Exhaust emission control device of internal combustion engine

ABSTRACT

An object of the present invention is to improve reliability of exhaust emission purification in an exhaust emission purifying device having an NOx catalyst and an bypass path bypassing the NOx catalyst. The present invention provides an exhaust emission purifying device including an SOx absorbing material  17  provided in an exhaust passage of an internal combustion engine, a main NOx catalyst  20  provided in the exhaust passage on the downstream side of the SOx absorbing material, a bypass pipe  26  branching off from the exhaust passage at a position between the SOx absorbing material  17  and the main NOx catalyst  20 , and an exhaust switching valve  28  provided at the start end of the bypass pipe  26  and adapted to switch the exhaust flow between the main NOx catalyst  20  and the bypass pipe  26 , wherein a sub NOx catalyst  24  is provided in the bypass pipe  26  and wherein when the exhaust switching valve  28  is controlled so as to lead the exhaust to the main NOx catalyst  20  and as to prevent the exhaust from flowing through the bypass pipe  26 , any exhaust leaking from the exhaust switching valve  28  to the bypass pipe  26  is purified by the sub NOx catalyst  24.

TECHNICAL FIELD

The present invention relates to an exhaust emission purifying devicecapable of removing nitrogen oxides (NOx) from the exhaust emitted froman internal combustion engine capable of lean burning.

BACKGROUND ART

Recently, as a vehicle-mounted internal combustion engine, a lean-burntype internal combustion engine is being developed which is capable ofburning an air-fuel mixture with excessive oxygen. Along with this, atechnology for removing harmful gas components, in particular, nitrogenoxides (NOx), contained in the exhaust gas from a lean-burn typeinternal combustion engine is being developed.

As a technology for purifying the exhaust emitted from a lean-burn typeinternal combustion engine, a technology is known according to which anNOx absorbing material, such as an occlusion reduction type NOxcatalyst, is provided in the exhaust passage of an internal combustionengine.

The NOx absorbing material absorbs the NOx in the exhaust when theair-fuel ratio of the exhaust flowing therethrough indicates excessiveoxygen (that is, in the case of a lean air-fuel ratio), and releases theNOx it has absorbed when the oxygen concentration of the exhaust flowingtherethrough is reduced. The occlusion reduction type NOx catalyst,which is an example of the NOx absorbing material, is a catalyst whichabsorbs the NOx in the exhaust when the air-fuel ratio of the exhaustflowing in is lean, and reduces the NOx to nitrogen (N₂) while releasingthe NOx it has absorbed when the oxygen concentration of the exhaustflowing in is reduced.

When the occlusion reduction type NOx catalyst is arranged in theexhaust passage of a lean-burn-type internal combustion engine, the NOxin the exhaust is absorbed by the occlusion reduction type NOx catalystwhen the air-fuel ratio of the exhaust is lean, and the NOx which hasbeen absorbed by the occlusion reduction type NOx catalyst is releasedas NO₂ when the air-fuel ratio of the exhaust is stoichiometric or rich,the NO₂ reacting with the reduction components in the exhaust, such ashydrocarbon (HC) and carbon monoxide (CO), to be thereby reduced tonitrogen (N₂)

In some cases, the fuel of an internal combustion engine contains sulfurcontent. If such a fuel is burnt in an internal combustion engine, thesulfur content in the fuel is oxidized to generate sulfur oxides (SOx),such as SO₂ and SO₃. The occlusion reduction type NOx catalyst absorbsthe SOx in the exhaust on the same principle as the absorption of NOx,so that, when the occlusion reduction type NOx catalyst is arranged inthe exhaust passage of an internal combustion engine, the occlusionreduction type NOx catalyst absorbs not only NOx but also SOx.

The SOx absorbed by the occlusion reduction type NOx catalyst forms astable sulfate with passage of time, so that, in the same conditions foreffecting release/reduction of NOx from the occlusion reduction type NOxcatalyst, it is not easily decomposed or released and tends to beaccumulated in the occlusion reduction type NOx catalyst. When theaccumulation amount of SOx in the occlusion reduction type NOx catalystincreases, the NOx absorption capacity of the occlusion reduction typeNOx catalyst decreases, and it becomes impossible to remove the NOx inthe exhaust to a sufficient degree, that is, so-called SOx poisoningoccurs.

To cope with this problem, there has conventionally been proposed anexhaust emission purifying device in which an SOx absorbing material forabsorbing the SOx contained in the exhaust is provided in the exhaustpassage on the upstream side of the occlusion reduction type NOxcatalyst. When the air-fuel ratio of the exhaust flowing in is lean, theSOx absorbing material absorbs the SOx in the exhaust, and when theair-fuel ratio of the exhaust flowing in is stoichiometric or rich, itreleases the SOx it has absorbed as SO₂.

In this exhaust emission purifying device, the SOx in the exhaust isremoved on the upstream side of the occlusion reduction type NOxcatalyst, and it is possible to prevent the SOx poisoning of theocclusion reduction type NOx catalyst.

However, there is a limitation to the SOx absorption capacity of the SOxabsorbing material, so that it is necessary to perform a processing forreleasing the SOx absorbed by the SOx absorbing material, that is, aregeneration processing, before the SOx absorption capacity of the SOxabsorbing material has been saturated.

An example of the SOx absorbing material regeneration technology isdisclosed in Japanese Patent No. 2605580. According to this patentofficial gazette, to release the SOx absorbed by the SOx absorbingmaterial, it is necessary for the air-fuel ratio of the exhaust flowingin to be stoichiometric or rich. Further, the higher the temperature ofthe SOx absorbing material, the easier it is for the SOx to be released.

In the exhaust emission purifying device disclosed in theabove-mentioned official gazette, to prevent the SOx released from theSOx absorbing material from being absorbed by the occlusion reductiontype NOx catalyst, there are provided an bypass path branching off fromthe exhaust pipe connecting the SOx absorbing material and the occlusionreduction type NOx catalyst and bypassing the occlusion reduction typeNOx catalyst, and an exhaust switching valve for selectively switchingthe exhaust flow between the occlusion reduction type NOx catalyst andthe bypass path. When executing the regeneration process of the SOxabsorbing material, the exhaust switching valve is controlled so as tocause all the exhaust from the SOx absorbing material to flow throughthe bypass path.

Further, in the exhaust emission purifying device disclosed in theabove-mentioned official gazette, when the regeneration process of theSOx absorbing material is not being performed, in other words, when theabsorption or releasing of NOx is to be performed by the occlusionreduction type NOx catalyst, the exhaust switching valve is controlledso as to cause all the exhaust to flow through the occlusion reductiontype NOx catalyst.

As is known in the art, the sealing property of the exhaust switchingvalve used in the above exhaust emission purifying device cannot beregarded as perfect, and the valve allows leakage of approximately 1 to10% of the exhaust. Thus, in the exhaust emission purifying devicedisclosed in the above official gazette, if the exhaust switching valveis controlled so as to allow the exhaust to flow through the occlusionreduction type catalyst and as to prevent the exhaust from flowing intothe bypass path, some exhaust is allowed to leak through the exhaustswitching valve to the bypass path, with the result that the NOxcontained in the exhaust leaking through the exhaust switching valve tothe bypass path is released to the atmosphere without being removed fromthe exhaust.

As a result of the recent progress in the catalyst technology, the NOxpurifying ratio by the occlusion reduction type NOx catalyst is over90%. Thus, the deterioration in exhaust emission control due to theleakage through the exhaust switching valve cannot be neglected.

Further, the above-described conventional exhaust emission purifyingdevice for an internal combustion engine is not provided with a meansfor reducing the hydrocarbon (HC) in the exhaust when the internalcombustion engine is started when the ambient temperature is low (thatis, at the time of low-temperature startup), so that there is the dangerof the hydrocarbon (HC) in the exhaust being released to the atmospherewithout being purified from the exhaust. Thus, the conventional exhaustemission purifying device leaves room for improvement.

DISCLOSURE OF THE INVENTION

The present invention has been made in view of the above variousproblems. It is an object of the present invention to prevent adeterioration in exhaust emission purifying due to leakage of theexhaust into the bypass path when the bypass path is closed by theexhaust flow switching means.

Another object of the present invention is to reduce the hydrocarbonconcentration in the exhaust when the internal combustion engine isstarted at low temperature.

To achieve the above objects, the present invention adopts the followingmeans.

An exhaust emission purifying device of an internal combustion engineaccording to the present invention comprises a lean-burn-type internalcombustion engine capable of burning an air-fuel mixture with excessiveoxygen, an NOx absorbing material which is arranged in an exhaustpassage of the internal combustion engine and which is adapted to absorbnitrogen oxides (NOx) when the air-fuel ratio of the exhaust flowing-inis lean and to release the nitrogen oxides (NOx) it has absorbed whenthe oxygen concentration of the exhaust flowing-in is low, a bypass pathbranching off from a portion of the exhaust passage on the upstream sideof the NOx absorbing material and allowing the exhaust to flow so as tobypass the NOx absorbing material, an exhaust flow switching means forselectively switching the exhaust flow between the NOx absorbingmaterial and the bypass path, an SOx absorbing material arranged in theexhaust passage on the upstream side of the exhaust flow switching meansand adapted to absorb sulfur oxides (SOx) when the air-fuel ratio of theexhaust flowing in is lean and to release the sulfur oxides (SOx) it hasabsorbed when the oxygen concentration of the exhaust flowing in is low,and an NOx catalyst provided in the bypass path and adapted to removethe nitrogen oxides (NOx) when the air-fuel ratio of the exhaust islean.

Normally, when purifying the exhaust discharged from an internalcombustion engine and, in particular, when purifying the nitrogen oxides(NOx) contained in the exhaust, the exhaust flow switching means iscontrolled such that the exhaust discharged from the internal combustionengine passes through the NOx absorbing material. In this case, althoughno exhaust ought to flow through the bypass path. However, when thesealing property of the exhaust flow switching means is not perfect, aminute amount of exhaust is allowed to leak to the bypass path throughthe exhaust flow switching means.

In contrast, in the exhaust emission purifying device for an internalcombustion engine of the present invention, the minute amount of exhaustleaking to the bypass path passes through the NOx catalyst provided inthe bypass path at a very low spatial velocity (hereinafter, spatialvelocity will be abbreviated as SV), so that the nitrogen oxides (NOx)contained in the exhaust leaking to the bypass path is efficientlypurified by the NOx catalyst.

As a result, in the exhaust emission purifying device for an internalcombustion engine of the present invention, even if some exhaust isallowed to flow through the bypass path when no exhaust should flowthrough the bypass path, the exhaust flowing through the by pass pathcan be released to the atmosphere after being purified, so that it isadvantageously possible to achieve an improvement in reliability inexhaust emission control.

Further, in the exhaust emission purifying device for an internalcombustion engine of the present invention, there is provided in theexhaust passage on the upstream side of the exhaust flow switching meansan SOx absorbing material adapted to absorb SOx when the air-fuel ratioof the exhaust is lean and to release the SOx it has absorbed when theoxygen concentration of the exhaust flowing in is low, so that the SOxin the exhaust is absorbed by the SOx absorbing material before theexhaust flows to the NOx absorbing material, whereby there is no dangerof the NOx absorbing material undergoing SOx poisoning.

In the exhaust emission purifying device for an internal combustionengine of the present invention, examples of an internal combustionengine capable of lean burning include an in-cylinder injection typelean burn gasoline engine and a diesel engine. In the case of a leanburn gasoline engine, the air-fuel ratio of the exhaust can becontrolled by controlling the air-fuel ratio of the air-fuel mixturesupplied to the combustion chamber. In the case of a diesel engine, theair-fuel ratio of the exhaust can be controlled by performing asecondary fuel injection during intake stroke, expansion stroke, orexhaust stroke, or by supplying a reducing agent to the exhaust passageon the upstream side of the NOx absorbing material. Here, the air-fuelratio of the exhaust is the ratio of the air to the fuel (hydrocarbon)supplied to the engine intake passage and to the portion of the exhaustpassage on the upstream side of the NOx absorbing member.

In the exhaust emission purifying device for an internal combustionengine of the present invention, examples of the NOx absorbing materialinclude an occlusion reduction type NOx catalyst. The occlusionreduction type NOx catalyst is a catalyst which absorbs the nitrogenoxides (NOx) in the exhaust when the air-fuel ratio of the exhaustflowing in is lean, and reduces the nitrogen oxides (NOx) it hasabsorbed to nitrogen (N₂) while releasing the nitrogen oxides when theoxygen concentration of the exhaust flowing in is low.

Examples of an occlusion reduction type NOx catalyst include a catalystcomprising an alumina carrier which carries thereon at least one of thefollowing metals: alkali metals, such as potassium K, sodium Na, lithiumLi, and cesium Cs; alkali earth metals, such as barium Ba and calciumCa; and rare earth metals, such as lanthanum La and yttrium Y, togetherwith a noble metal, such as platinum Pt.

In the exhaust emission purifying device for an internal combustionengine of the present invention, the exhaust flow switching means may beformed by a single switching valve provided in the branching portion ofthe bypass path, or by providing a first opening/closing valve in theexhaust passage at a position nearer to the NOx absorbing material thanto the branching portion and a second opening/closing valve in thebypass path.

In the exhaust emission purifying device for an internal combustionengine of the present invention, examples of the SOx absorbing materialinclude a material comprising a carrier consisting alumina and carryingthereon at least one of the following metals: transition metals such ascopper Cu, iron Fe, manganese Mn, and nickel Ni; and sodium Na; titaniumTi; and lithium Li. In order that the SOx may be easily absorbed by theSOx absorbing material in the form of sulphate ions SO₄ ²⁻, it isdesirable that one of platinum Pt, palladium Pd, and rhodium Rh becarried on the carrier of the SOx absorbing material.

In the exhaust emission purifying device for an internal combustionengine of the present invention, examples of the NOx catalyst providedin the bypass path include a selective reduction type NOx catalyst whichreduces or decomposes the nitrogen oxides (NOx) when hydrocarbon existsin an atmosphere with excessive oxygen. The selective reduction type NOxcatalyst generally exhibits its characteristics of providing a high NOxpurification ratio even with respect to a small amount of hydrocarbon(HC) when the exhaust flows at a low SV, and is capable of purifyinghydrocarbon (HC) or nitrogen oxides at a purifying ratio of 70 to 80%.

Thus, in the case in which the exhaust flow switching means iscontrolled such that the exhaust emitted from the internal combustionengine flows through the NOx absorbing material, when a minute amount ofexhaust leaks to the bypass path from the exhaust flow switching means,the minute amount of exhaust flows through the NOx catalyst at a low SVso that the nitrogen oxides (NOx), hydrocarbon (HC), etc. contained inthe exhaust are efficiently purified. Examples of the selectivereduction type NOx catalyst include a catalyst comprising a zeolitecarrier carrying platinum (Pt) thereon.

The NOx catalyst provided in the bypass path may be an occlusionreduction type NOx catalyst which absorbs nitrogen oxides (NOx) when theair-fuel ratio of the exhaust flowing in is lean and which reduces andpurifies the nitrogen oxides (NOx) it has absorbed while releasing thesame when the oxygen concentration of the exhaust flowing in is reducedand there exists a reducing agent such as hydrocarbon (HC).

Examples of the occlusion reduction type NOx catalyst include a catalystcomprising the alumina carrier which carries thereon at least one of thefollowing metals: alkali metals, such as potassium K, sodium Na, lithiumLi, and cesium Cs; alkali earth metals, such as barium Ba and calciumCa; and rare earth metals, such as lanthanum La and yttrium Y; togetherwith a noble metal, such as platinum Pt.

In the exhaust emission purifying device for an internal combustionengine of the present invention, it is desirable that the exhaust flowswitching means be controlled so as to permit the exhaust to flow to theNOx absorbing material and to inhibit the exhaust to flow to the bypasspath when the air-fuel ratio is controlled to be lean, and to permit theexhaust to flow to the bypass path and inhibit the exhaust to flow tothe NOx absorbing material when the air-fuel ratio is controlled to bestoichiometric or rich.

When the air-fuel ratio of the exhaust is controlled to be lean, sulfuroxides (SOx) contained in the exhaust are absorbed by the SOx absorbingmaterial, and the exhaust from which the sulfur oxides (SOx) have beenremoved flows through the NOx absorbing material, so that only thenitrogen oxides (NOx) in the exhaust are absorbed by the NOx absorbingmaterial, and it is possible to reliably prevent occurrence of so-calledSOx poisoning, in which sulfur oxides (SOx) are absorbed by the NOxabsorbing material.

On the other hand, when the air-fuel ratio of the exhaust is controlledto be stoichiometric or rich, the exhaust passed through the SOxabsorbing material is discharged by way of the bypass path, and noexhaust is allowed to flow through the NOx absorbing material, so that,even if sulfur oxides (SOx) are released from the SOx absorbingmaterial, the sulfur oxides (SOx) released from the SOx absorbingmaterial do not flow into the NOx absorbing material, and there is nodanger of the NOx absorbing material undergoing SOx poisoning. Since theexhaust having the air-fuel ratio of stoichiometric or rich flowsthrough the NOx catalyst in the bypass path, the sulfuroxides (SOx)adsorbed by the NOx catalyst are released to become SO₂.

Here, the expression: “when the air-fuel ratio of the exhaust iscontrolled to be stoichiometric or rich” is of course the concept whichcovers the case in which the air-fuel ratio of the exhaust is controlledto be stoichiometric or rich in order to perform the regenerationprocessing on the SOx absorbing material, but also covers the case inwhich the air-fuel ratio of the exhaust becomes stoichiometric or richas a result of the air-fuel ratio of the air-fuel mixture beingcontrolled to be stoichiometric or rich according to the operatingcondition of the engine.

Examples of the engine operating condition in which the air-fuel ratioof the air-fuel mixture is stoichiometric or rich include high-loadoperating condition, a full-load operating condition, and warming upoperation after engine startup.

In the exhaust emission purifying device for an internal combustionengine of the present invention, the NOx catalyst provided in the bypasspath may have a 3-way purifying function and an HC adsorption capacityat a low temperature. In this case, it is desirable for the exhaust flowswitching means to be controlled such that when the temperature of theexhaust is less than a predetermined temperature, the exhaust is led tothe bypass path and is prevented from flowing into the NOx absorbingmaterial, and that when the temperature of the exhaust is higher thanthe predetermined temperature, the exhaust is led to the NOx absorbingmaterial and is prevented from passing through the bypass path.

When the temperature of the exhaust is less than the predeterminedtemperature, the NOx absorbing material is not activated yet, so that itis impossible to sufficiently purified the exhaust at this temperatureby passing it through the NOx absorbing material. In the exhaustemission purifying device of the present invention, when the exhausttemperature is less than the predetermined temperature, the exhaust iscaused to flow through the bypass path, whereby the hydrocarbon (HC) inthe exhaust is adsorbed by the NOx catalyst.

As a result, when the internal combustion engine is started at a lowtemperature, the exhaust is advantageously released to the atmosphereafter being purified.

On the other hand, when the temperature of the exhaust has been raisedto a level higher than the predetermined temperature, the NOx absorbingmaterial is activated, and can exert the purifying capacity, so that theexhaust flow switching means is controlled such that the exhaust is ledto the NOx absorbing material and that the exhaust is prevented fromflowing through the bypass path.

At this time, if the sealing property of the exhaust flow switchingmeans is not perfect, a minute amount of exhaust leaks from the exhaustflow switching means to the bypass path. However, since the amount ofexhaust leaking from the exhaust flow switching means to the bypass pathis relatively small, the minute amount of exhaust flows through the NOxcatalyst provided in the bypass path at a low SV.

When the exhaust flows through the NOx catalyst at a low SV, thereaction of the nitrogen oxides (NOx) contained in the exhaust with thehydrocarbon (HC) adsorbed by the NOx catalyst is promoted, so that thenitrogen oxides (NOx) in the exhaust are effectively purified, and animprovement is achieved in terms of exhaust emission purification.Further, as stated above, the hydrocarbon (HC) adsorbed by the NOxcatalyst is consumed as the reducing agent for the nitrogen oxides(NOx), and in addition, it reacts with the oxygen contained in theexhaust to be thereby purified, so that the exhaust emissionpurification is further improved.

In the exhaust emission purifying device for an internal combustionengine of the present invention, when the internal combustion engine isan in-cylinder injection type internal combustion engine provided with afuel injection valve for directly injecting fuel into the combustionchamber of the internal combustion engine, and the SOx absorbingmaterial has a 3-way purifying function, the exhaust flow switchingmeans may be controlled at the startup of the internal combustion engineso as to throttle the exhaust flow amount passing through the NOxabsorbing material and the NOx catalyst and as to cause the fuelinjection valve to perform a secondary fuel injection during theexpansion stroke of each cylinder in addition to the injection of thefuel for combustion.

When, at the startup of the internal combustion engine, the exhaust flowpassing through the NOx absorbing material and the NOx catalyst isthrottled, the back pressure acting on the internal combustion enginerises to cause the temperature of the exhaust to rise. When, in thiscondition, the fuel is injected secondarily from the fuel injectionvalve during the expansion stroke of each cylinder, the reaction of theinjected fuel with the oxygen in the exhaust is promoted. When thereaction of the fuel with the oxygen is promoted, the quantity of heatgenerated at the time of the reaction of the fuel and oxygen increases,and the exhaust temperature rises. When the exhaust, which has thusattained high temperature, flows into the SOx absorbing material, theheat of the exhaust is transmitted to the SOx absorbing material, andthe temperature of the SOx absorbing material rises abruptly, with theresult that the 3-way purifying function of the SOx absorbing materialis activated at an early stage. As a result, it is possible to improvethe exhaust emission purification when the internal combustion engine isstarted at a low temperature.

In the exhaust emission purifying device for an internal combustionengine of the present invention, it is also possible to further providea temperature rise restraining means which controls the exhaust flowswitching means such that the exhaust flows through both the NOxabsorbing material and the NOx catalyst when the temperature of the NOxabsorbing material becomes higher than a predetermined temperature whenthe exhaust flow switching means is being controlled such that theexhaust is led to the NOx absorbing material and the exhaust isprevented from flowing into the NOx catalyst.

The NOx absorbing material has such characteristics that, when it is ina predetermined temperature range, efficiently absorbs the nitrogenoxides (NOx), so that when the exhaust temperature becomes higher than apredetermined temperature when the entire amount of exhaust is flowingthrough the NOx absorbing material, the temperature of the NOx absorbingmaterial exceeds the activation temperature range, with the result thatit becomes difficult for the NOx absorbing material to absorb nitrogenoxides (NOx). In view of this, the temperature rise restraining means socontrols the exhaust flow switching means that the exhaust flows throughboth the NOx absorbing material and the NOx catalyst.

In this case, the amount of exhaust flowing through the NOx absorbingmaterial is reduced by half as compared with the case in which theentire amount of exhaust flows through the NOx absorbing material, sothat the quantity of heat the NOx absorbing material receives from theexhaust is also reduced by half, and there is no excessive temperaturerise of the NOx absorbing material, with its temperature being keptwithin the activation temperature range.

When the entire amount of exhaust is flowing through the NOx absorbingmaterial, the air-fuel ratio of the exhaust is controlled to be lean, sothat it is preferable that the NOx catalyst is one which functions topurify the NOx in the exhaust when the air-fuel ratio of the exhaust islean. Examples of such a NOx catalyst include an occlusion reductiontype NOx catalyst.

The temperature rise restraining means may be one which executes the SOxpoisoning regeneration processing on the NOx catalyst immediately beforecontrolling the exhaust flow switching means to cause the exhaust toflow through both the NOx absorbing material and the NOx catalyst.

In the exhaust emission purifying device for an internal combustionengine of the present invention, when the air-fuel ratio of the exhaustis being controlled to be stoichiometric or rich, the exhaust flowswitching means is controlled such that the entire amount of the exhaustflows through the NOx catalyst, so that, in this process, it is assumedthat the sulfur oxides (SOx) released from the SOx absorbing material isadsorbed by the NOx catalyst to thereby cause SOx poisoning. At the sametime, when the exhaust flows through both the NOx absorbing material andthe NOx catalyst in the condition in which the NOx catalyst hasundergone SOx poisoning, it is assumed that the NOx purifying ratio ofthe NOx catalyst will be reduced.

In the exhaust emission purifying device for an internal combustionengine of the present invention, the exhaust flow switching means may becontrolled such that when the internal combustion engine is performingwarming-up operation, the exhaust is led to the NOx catalyst and thatthe exhaust is prevented from flowing into the NOx absorbing material,switching being performed such that after the completion of the warmingup of the internal combustion engine, the exhaust is led to the NOxabsorbing material and is prevented from flowing into the NOx catalystat the time when the NOx exhaust amount from the internal combustionengine has become less than the predetermined amount.

When the internal combustion engine is the warming up state, theair-fuel ratio of the exhaust is controlled to be stoichiometric orrich, so that the exhaust flow switching means is controlled so as tocause the entire amount of exhaust to flow through the NOx catalyst toprevent the sulfur oxides (SOx) released from the SOx absorbing materialfrom flowing into the NOx absorbing material. Thus, no exhaust flowsthrough the NOx absorbing material until the warming up of the internalcombustion engine is completed and the operating condition of theinternal combustion engine has been switched to the lean air-fuel ratiooperation, and the NOx absorbing material is expected to be in thenon-activated state even after the completion of the warming up of theinternal combustion engine. In such a case, when the exhaust flowswitching means is controlled such that the entire amount of exhaustflows through the NOx absorbing material, the nitrogen oxides (NOx) inthe exhaust are not purified by the NOx absorbing material, and there isthe danger of deteriorating the exhaust emission purification.

In contrast, in the exhaust emission purifying device for an internalcombustion engine of the present invention, when, after the completionof the warming up of the internal combustion engine, the amount ofnitrogen oxides (NOx) discharged from the internal combustion enginebecomes less than the predetermined amount, the exhaust flow switchingmeans is switched from the state in which the entire amount of exhaustflows through the NOx catalyst to the state in which the entire amountof exhaust flows through the NOx absorbing material.

In this case, after the completion of the warming up of the internalcombustion engine, the exhaust gas flows into an NOx absorbing materialin the non-activated state, so that the temperature of the NOx absorbingmaterial rises due to the heat of the exhaust. In this process, theexhaust flows through the NOx absorbing material in the non-activatedstate, but, since the amount of nitrogen oxides (NOx) contained in theexhaust is very small, it is possible to raise the temperature of theNOx absorbing material while restraining the deterioration in exhaustemission to a minimum.

The amount of nitrogen oxides (NOx) discharged from the internalcombustion engine becomes less than the predetermined amount when, forexample, the vehicle on which the internal combustion engine is mountedis running at decelerated speed, or when the load of the internalcombustion engine becomes less than a predetermined value, and, in thisregard, the so-called fuel cut condition is preferable, in which thefuel injection is stopped in the internal combustion engine.

Further, while, at the time of warming up of the internal combustionengine, the air-fuel ratio of the exhaust is being controlled to bestoichiometric or rich, the exhaust flow switching means may becontrolled such that the exhaust is led to the Nox catalyst and that theexhaust is prevented from flowing into the NOx absorbing material, andwhile, at the warming up of the internal combustion engine, the amountof NOx discharged from the internal combustion engine is less than thepredetermined amount, it may be controlled such that the exhaust is ledto the NOx absorbing material and that the exhaust is prevented fromflowing into the NOx catalyst.

In this case, it is possible to activate the NOx absorbing materialwhile restraining the deterioration of exhaust emission at the time ofwarming up of the internal combustion engine. As a result, it ispossible to improve the exhaust emission when the exhaust starts to flowthrough the NOx absorbing material after the completion of the warmingup of the internal combustion engine.

In the exhaust emission purifying device for an internal combustionengine of the present invention, when SOx poisoning of at least one ofthe NOx absorbing material and the NOx catalyst is detected, it ispossible to control the exhaust flow switching means to such that theexhaust flows through both the NOx absorbing material and the NOxcatalyst, and to further provide an SOx poisoning regeneration means forsimultaneously executing a SOx poisoning regeneration processing on theNOx absorbing material and the NOx catalyst.

When simultaneously performing the SOx poisoning regeneration on the NOxabsorbing material and the NOx catalyst, the frequency of execution ofthe SOx poisoning regeneration processing is reduced as compared withthe case in which the SOx poisoning regeneration is individually andseparately performed on the NOx absorbing material and the NOx catalyst.In the SOx poisoning regeneration processing, it is necessary to raisethe temperature of the NOx absorbing material and the NOx catalyst to arelatively high temperature range, so that the fuel is burnt in the NOxabsorbing material and the NOx catalyst. Thus, when the frequency ofexecution of the SOx poisoning regeneration processing is reduced, thefuel consumption amount related to the SOx poisoning regenerationprocessing is reduced. Further, when in the SOx poisoning regenerationprocessing the exhaust flows through both the NOx absorbing material andthe NOx catalyst, the SV of the exhaust in the NOx absorbing materialand the NOx catalyst is reduced, whereby the SOx purification ratio isimproved.

In the exhaust emission purifying device for an internal combustionengine of the present invention, it is also possible to provide, inaddition to the SOx poisoning regeneration processing means, aregeneration completion determination means for determining the SOxpoisoning regeneration completion of the NOx absorbing material and theNOx catalyst. In this case, when it is determined by the regenerationcompletion determination means that the SOx poisoning regeneration ofone of the NOx absorbing material and the NOx catalyst has beencompleted, the SOx poisoning regeneration means may control the exhaustflow switching means so as to prevent the exhaust from flowing to thesubstance on which the SOx poisoning regeneration has been completed.

The reason for this control is that, when the exhaust is continued to besupplied to the NOx absorbing material or the NOx catalyst on which SOxpoisoning regeneration processing has been completed, the fuel componentcontained in the exhaust is burnt in the NOx absorbing material or theNOx catalyst to cause an unnecessary rise in the temperature of the NOxabsorbing material or the NOx catalyst, thereby causing a heatdeterioration in the NOx absorbing material or the NOx catalyst.

Further, when it is determined by the regeneration completiondetermination means that the SOx poisoning regeneration of one of theNOx absorbing material and the NOx catalyst has been completed, the SOxpoisoning regeneration means may interrupt the SOx poisoningregeneration processing and cool the one on which the SOx poisoningregeneration has been completed, resuming the SOx poisoning regenerationprocessing solely on the one on which the SOx poisoning regeneration hasnot been completed yet after the completion of the cooling of the one onwhich the SOx poisoning regeneration has been completed.

In this case, the NOx absorbing material or the NOx catalyst on whichthe SOx poisoning regeneration has been completed is not left at a hightemperature, whereby it is possible to further improve the durability ofthe NOx absorbing material and the NOx catalyst.

Next, in the exhaust emission purifying device for an internalcombustion engine of the present invention, when the NOx catalystconsists of an occlusion reduction type NOx catalyst, it is possible tofurther provide an NOx absorption amount detection means for detectingthe amount of nitrogen oxides (NOx) absorbed by the NOx absorbingmaterial and the amount of nitrogen oxides (NOx) absorbed by the NOxcatalyst.

When the NOx catalyst consists of an occlusion reduction type NOxcatalyst, NOx are absorbed by the NOx catalyst on the same principle asthat of the NOx absorbing material, so that it is necessary to releaseand purify the nitrogen oxides (NOx) absorbed by the NOx catalyst beforethe nitrogenoxide (NOx) absorbing capacity of the NOx catalyst issaturated. In view of this, in the exhaust emission purifying device ofthe present invention, there is provided an NOx absorption amountdetection means capable of detecting the amount of nitrogen oxides (NOx)absorbed by the NOx catalyst in addition to the amount of nitrogenoxides (NOx) absorbed by the NOx absorbing material.

It is preferable that the NOx absorption amount detection meansestimates the amount of nitrogen oxides (NOx) absorbed by each of theNOx absorbing material and the NOx catalyst on the basis of the amountof exhaust leaking from the exhaust flow switching means. In this case,it is possible to estimate the absorption amounts of nitrogen oxides(NOx) of the NOx absorbing material and the NOx catalyst with highaccuracy, whereby it is possible to accurately set the execution timefor releasing and purifying the nitrogen oxides (NOx).

In the exhaust emission purifying device for an internal combustionengine of the present invention, when it is necessary to control theexhaust flow switching means such that the exhaust flows through boththe NOx absorbing material and the NOx catalyst, it is possible tofurther provide an NOx purifying means for controlling the exhaust flowswitching means such that the exhaust flows through both the NOxabsorbing material and the NOx catalyst after all the nitrogen oxides(NOx) absorbed by the NOx absorbing material and the NOx catalyst havebeen released and purified.

This means is provided for the case in which the NOx absorbing capacityof the NOx absorbing material is the same as the NOx absorbing capacityof the NOx catalyst. In this case, the nitrogen oxide (NOx) absorptionamount of the NOx absorbing material and the nitrogen oxide (NOx)absorption amount of the NOx catalyst are set to zero before the exhaustflows through both the NOx absorbing material and the NOx catalyst, sothat the time when the nitrogen oxide (NOx) absorbing capacity of theNOx absorbing material is saturated is the same as the time when thenitrogen oxide (NOx) absorbing capacity of the NOx catalyst issaturated. As a result, the nitrogen oxide (NOx) releasing/purifyingprocessing for the NOx absorbing material is performed at the same timeas the nitrogen oxide (NOx) releasing/purifying processing for the NOxcatalyst, with the result that the execution frequency of the nitrogenoxide (NOx) releasing/purifying processing decreases, whereby it ispossible to reduce the fuel consumption amount related to the NOxreleasing/purifying processing.

On the other hand, when the nitrogen oxide (NOx) absorbing cap capacityof the NOx absorbing material is different from the nitrogen oxide (NOx)absorbing capacity of the NOx catalyst, all the nitrogen oxides (NOx)absorbed by the NOx absorbing material and the NOx catalyst are releasedand purified before the exhaust flow switching means is controlled suchthat the exhaust flows through both the NOx absorbing material and theNOx catalyst. And, when the exhaust flow switching means is controlledsuch that the exhaust flows through both the NOx absorbing material andthe NOx catalyst, the NOx purifying means simultaneously releases andpurifies the nitrogen oxides (NOx) absorbed by the NOx absorbingmaterial and the nitrogen oxides (NOx) absorbed by the NOx catalyst,using, of the NOx absorbing material and the NOx catalyst, the one whoseNOx absorbing capacity is less as a reference.

The exhaust emission purifying device for an internal combustion engineof the present invention is arranged in the exhaust passage of alean-burn type internal combustion engine, and may comprise an NOxabsorbing material which absorbs nitrogen oxides (NOx) when the air-fuelratio of the exhaust flowing-in is lean and which releases the nitrogenoxides (NOx) it has absorbed when the oxygen concentration of theexhaust flowing-in is low, a bypass path branching off from the exhaustpassage on the upstream side of the NOx absorbing material and allowingthe exhaust to flow so as to bypass the NOx absorbing material, anexhaust flow switching means selectively switching the exhaust flowbetween the NOx absorbing material and the bypass path, an SOx absorbingmaterial arranged in the exhaust passage on the upstream side of theexhaust flow switching means and adapted to absorb sulfur oxides (SOx)when the air-fuel ratio of the exhaust flowing-in is lean and to releasethe sulfur oxides (SOx) it has absorbed when the oxygen concentration ofthe exhaust flowing-in is low, and a NOx catalyst provided in theexhaust passage on the downstream side of the bypass path and adapted topurify the nitrogen oxides (NOx) when the air-fuel ratio of the exhaustis lean.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing the construction of a firstembodiment of an exhaust emission purifying device for an internalcombustion engine of the present invention;

FIG.2 is a diagram showing an example of the basic fuel injection timemap;

FIG. 3 is a chart schematically showing the concentrations of theunburned HC, CO and oxygen in the exhaust discharged from the internalcombustion engine;

FIG. 4 is a diagram illustrating the NOx absorbing/releasing action ofan occlusion reduction type NOx catalyst;

FIG. 5 is a diagram showing an example of the air-fuel ratio control inthe first embodiment;

FIG. 6 is a flowchart showing the exhaust flow switching processingexecution routine of the first embodiment;

FIG. 7 is a flowchart showing the exhaust flow switching processingexecution routine in a second embodiment of an exhaust emissionpurifying device for an internal combustion engine of the presentinvention;

FIG. 8 is a schematic diagram showing the construction of a thirdembodiment of an exhaust emission purifying device for an internalcombustion engine of the present invention;

FIG. 9 is a diagram showing in detail the construction of an exhaustmanifold in the third embodiment;

FIG. 10 is a flowchart showing the normal-time exhaust switching controlroutine in the third embodiment;

FIG. 11 is a flowchart showing the catalyst temperature rise controlroutine in the third embodiment;

FIG. 12 is a flowchart showing the main NOx catalyst temperature risecontrol routine in a fourth embodiment;

FIG. 13 is a flowchart showing a SOx poisoning regeneration controlroutine in a fifth embodiment;

FIG. 14(A) is a flowchart (1) showing an SOx poisoning regenerationcontrol routine in a sixth embodiment;

FIG. 14(B) is a flowchart (2) showing the SOx poisoning regenerationcontrol routine in the sixth embodiment;

FIG. 15 is a flowchart showing a NOx catalyst temperature riserestraining control routine in a seventh embodiment;

FIG. 16(A) is a flowchart (1) showing the rich spike control routine inan eighth embodiment;

FIG. 16(B) is a flowchart (2) showing the rich spike control routine inthe eighth embodiment;

FIG. 17(A) is a flowchart (1) showing the rich spike control routine ina ninth embodiment;

FIG. 17(B) is a flowchart (2) showing the rich spike control routine inthe ninth embodiment;

FIG. 18 is a flowchart showing the main NOx catalyst temperature risecontrol routine in a tenth embodiment; and

FIG. 19 is a diagram showing the hardware construction of an exhaustemission purifying device for an internal combustion engine according toanother embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the exhaust emission purifying device for an internalcombustion engine according to the present invention will now bedescribed with reference to FIGS. 1 through 19.

First Embodiment

FIG. 1 is a diagram schematically showing the construction of thepresent invention when it is applied to a vehicle gasoline enginecapable of lean-burn combustion. In the drawing, numeral 1 indicates aninternal combustion engine main body, numeral 2 indicates a piston,numeral 3 indicates a combustion chamber, numeral 4 indicates a sparkingplug, numeral 5 indicates an intake valve, numeral 6 indicates an intakeport, numeral 7 indicates an exhaust valve, and numeral 8 indicates anexhaust port.

The intake port 6 is connected to a surge tank 10 through each branchpipe of an intake manifold 9, and a fuel injection valve 11 forinjecting the fuel to the intake port 6 is mounted to each branch pipeof the intake manifold 9. The surge tank 10 is connected to an aircleaner 14 through an intake duct 12 and an airflow meter 13, and athrottle valve 15 is arranged in the intake duct 12.

On the other hand, the exhaust port 8 is connected to a casing 18containing an SOx absorbing material 17 through an exhaust manifold 16,and the outlet portion of the casing 18 is connected to a casing 21containing an occlusion reduction type NOx catalyst (NOx absorbingmaterial) 20 through an exhaust pipe 19. In the following, thisocclusion reduction type NOx catalyst 20 will be referred to as the mainNOx catalyst 20. The casing 21 is connected to an unillustrated mufflerthrough an exhaust pipe 22.

An inlet pipe portion 21 a of the casing 21 and the exhaust pipe 22 arealso connected by a bypass path 26 bypassing the main NOx catalyst 20.The bypass path 26 is formed by a bypass pipe 26A connected to the inletpipe portion 21 a of the casing 21, a bypass pipe 26B connected to theexhaust pipe 22, and a casing 23 placed between the bypass pipes 26A and26B, and a selective reduction type NOx catalyst 24 is accommodated inthe casing 23. In the following, this selective reduction type NOxcatalyst 24 will be referred to as the sub NOx catalyst 24. In thisembodiment, the sub NOx catalyst 24 consists of platinum (Pt) carried byzeolite, and exerts sufficient 3-way purifying function when theair-fuel ratio of the exhaust flowing-in is stoichiometric.

Provided in the inlet pipe portion 21 a of the casing 21, which is abranching portion of the bypass pipe 26A, is an exhaust switching valve(exhaust flow switching means) 28 whose valve body is operated by anactuator 27. This exhaust switching valve 28 is operated by the actuator27 to select between a bypass-closed position in which the inlet portionof the bypass pipe 26A is closed as indicated by the solid line of FIG.1 and in which the inlet pipe portion 21 a of the casing 21 is fullyopen, and a bypass-open position in which the inlet pipe portion 21 a ofthe casing 21 is closed as indicated by the broken line of FIG. 1 and inwhich the inlet portion of the bypass pipe 26A is fully open.

An electronic control unit (ECU) 30 for controlling the engine comprisesa digital computer, and is provided with a ROM (read only memory) 32, aRAM (random access memory) 33, a CPU (central processor unit) 34, aninput port 35, and an output port 36, which are connected to each otherby a bidirectional bus 31. The airflow meter 13 generates an outputvoltage in proportion to the intake air amount, and this output voltageis input into an input port 35 through a corresponding A/D converter 38.Further, mounted to the throttle valve 15 is an idle switch 40 fordetecting that the throttle valve 15 is at an idling opening degree, andan output signal of this idle switch 40 is input into the input port 35.

On the other hand, in the exhaust pipe 19 on the downstream side of theSOx absorbing material 17, there is mounted a temperature sensor 29 forgenerating an output voltage in proportion to the temperature of theexhaust which has passed the SOx absorbing material 17, and the outputvoltage of this temperature sensor 29 is input into the input port 35through the A/D converter 38. Further, connected to the input port 35 isan RPM sensor 41 adapted to generate an output pulse indicating theengine RPM. The output port 36 is connected to the sparking plug 4, thefuel injection valve 11, and the actuator 27 through correspondingdriving circuits 39.

In this internal combustion engine 1, the fuel injection time TAU iscalculated based, for example, on the following equation:

TAU=TP·K

Here, TP represents the basic fuel injection time, and K represents acorrection coefficient. The basic fuel injection time TP indicates therequired fuel injection time for making the air-fuel ratio of theair-fuel mixture supplied to the cylinder of the internal combustionengine 1 stoichiometric. This basic fuel injection time TP is obtainedin advance by experiment, and is stored in advance in the ROM 32 in theform of a map as shown in FIG. 2 as a function of the engine load Q/N(intake-air-amount Q/engine-RPM N) and the engine RPM N.

The correction coefficient K is a coefficient for controlling theair-fuel ratio of the air-fuel mixture supplied to the cylinder of theinternal combustion engine 1. When K=1.0, the air-fuel ratio of theair-fuel mixture supplied to the cylinder is stoichiometric. On theother hand, when K<1.0, the air-fuel ratio of the air-fuel mixturesupplied to the cylinder is higher than the stoichiometric ratio, thatis, lean, and when K>1.0, the air-fuel ratio of the air-fuel mixturesupplied to the cylinder is less than the stoichiometric ratio, that is,rich.

In this embodiment, when the engine operating condition is in alow-middle load operating range, the value of the correction coefficientK is smaller than 1.0, and the internal combustion engine 1 is operatedat a lean air-fuel ratio. When the engine operating condition is in ahigh-load operating range, when the engine operating condition is in awarming-up operating range after startup, when the engine operatingcondition is in an accelerating operation range, and when the internalcombustion engine is in such an engine operating condition that thevehicle in which the internal combustion engine 1 is mounted makes anormal run at a predetermined speed (for example, higher than 120 km/h)(normal operating condition), the value of the correction coefficient Kis 1.0, and the internal combustion engine is operated at thestoichiometric air-fuel ratio. When the engine operating condition is inthe full-load operating range, the value of the correction coefficient Kis larger than 1.0, and the internal combustion engine 1 is operated ata rich air-fuel ratio.

In the following, the control operation for controlling the fuelinjection amount so as to operate the internal combustion engine 1 at alean air-fuel ratio is referred to as the lean air-fuel ratio control;the control operation for controlling the fuel injection amount so as tooperate the internal combustion engine 1 at a stoichiometric air-fuelratio is referred to as the stoichiometric control; and the controloperation for controlling the fuel injection amount so as to operate theinternal combustion engine 1 at a rich air-fuel ratio is referred to asthe rich air-fuel ratio control.

Normally, in a gasoline engine like the internal combustion engine 1,the low-middle load operation is performed at the highest frequency, sothat in the major part of the operation period, the value of thecorrection coefficient K is smaller than 1.0, and a air-fuel mixturehaving the lean air-fuel ratio is burnt.

FIG. 3 schematically shows the concentrations of the main components ofthe exhaust discharged from the combustion chamber 3. As can be seenfrom this diagram, the lower the air-fuel ratio of the air-fuel mixturesupplied to the combustion chamber 3 (i.e., the richer the air-fuelmixture), the higher the concentrations of the unburned HC and CO in theexhaust discharged from the combustion chamber 3, and the higher theair-fuel ratio of the air-fuel mixture supplied to the combustionchamber 3 (i.e., the leaner the air-fuel mixture), the higher theconcentration of the oxygen O₂ in the exhaust discharged from thecombustion chamber 3.

The main NOx catalyst 20 accommodated in the casing 21 comprises, forexample, alumina, as the carrier, which carries thereon at least one ofthe following metals: alkali metals, such as potassium K, sodium Na,lithium Li, and cesium Cs; alkali earth metals, such as barium Ba andcalcium Ca; and rare earth metals, such as lanthanum La and yttrium Y,together with a noble metal such as platinum Pt. The ratio of the air tothe fuel (hydrocarbon) supplied to the intake passage of the internalcombustion engine 1 and the exhaust passage on the upstream side of themain NOx catalyst 20 will be referred to as the air-fuel ratio of theexhaust flowing into the main NOx catalyst 20 (hereinafter abbreviatedas the exhaust air-fuel ratio) Then, this main NOx catalyst 20 absorbsNOx when the exhaust air-fuel ratio is lean, and releases the NOx it hasabsorbed when the oxygen concentration of the exhaust flowing-in is low.

When fuel (hydrocarbon) or air is not supplied to the exhaust passage onthe upstream side of the main NOx catalyst 20, the exhaust air-fuelratio coincides with the air-fuel ratio of the air-fuel mixture suppliedto the combustion chamber 3. Thus, in this case, the main NOx catalyst20 absorbs NOx when the air-fuel ratio of the air-fuel mixture suppliedto the combustion chamber 3 is lean, and releases the NOx it hasabsorbed when the oxygen concentration of the air-fuel mixture suppliedto the combustion chamber 3 is reduced.

When the above-described main NOx catalyst 20 is arranged in the exhaustpassage of the internal combustion engine 1, this main NOx catalyst 20actually performs absorption and releasing of NOx. The mechanism of theabsorption and releasing is to be regarded as shown in FIG. 4. In thefollowing, this mechanism will be described with reference to the case,as an example, in which platinum Pt and barium Ba are carried on thecarrier. The mechanism is the same if some other noble metal and someother alkali metal, alkali earth metal, or rare earth metal are used.

First, when the exhaust flowing -in becomes leaner (i.e., when theair-fuel ratio becomes higher), the oxygen concentration of the exhaustflowing-in increases substantially, and, as shown in FIG. 4(A), theoxygen O₂ adheres to the surface of the platinum Pt in the form of O₂ ⁻or O²⁻. On the other hand, the NO contained in the exhaust flowing-inreacts with the O₂ ⁻ or O²⁻ on the surface of the platinum Pt to becomeNO₂ (2NO+O₂→2NO₂).

Next, part of the NO₂ produced is absorbed by the main NOx catalyst 20while being oxidized on the platinum Pt, and, as shown in FIG. 4 (A), isdiffused into the main NOx catalyst 20 in the form of nitrate ions NO₃ ⁻while being combined with barium oxide BaO. In this way, NOx is absorbedby the main NOx catalyst 20.

As long as the oxygen concentration of the exhaust flowing-in is high,NO₂ is produced on the surface of the platinum Pt, and, as long as theNOx absorbing capacity of the main NOx catalyst 20 is not saturated, NO₂is absorbed by the main NOx catalyst 20 to produce nitrate ions NO₃ ⁻.

On the other hand, when the oxygen concentration of the exhaustflowing-in is reduced and the amount of NO₂ generated is reduced, thereaction is reversed (NO₃ ⁻→NO₂), and the nitrate ions NO₃ ⁻ in the mainNOx catalyst 20 are released from the main NOx catalyst 20 in the formof NO₂ or NO. That is, when the oxygen concentration of the exhaustflowing-in is reduced, NOx is released from the main NOx catalyst 20. Asshown in FIG. 3, when the degree of leanness of the exhaust flowing-inis low (i.e., when the air-fuel ratio is low), the oxygen concentrationof the exhaust flowing-in is reduced, so that when the degree ofleanness of the exhaust flowing-in is low, NOx is released from the mainNOx catalyst 20.

On the other hand, when, at this time, the fuel injection amount of theinternal combustion engine 1 undergoes stoichiometric control or richair-fuel ratio control and the exhaust air-fuel ratio becomesstoichiometric or rich, a large amount of unburned HC and CO isdischarged from the internal combustion engine 1 as shown in FIG. 3, andthese unburned HC and CO react with the O₂ ⁻ or O²⁻ on the platinum Ptto be oxidized.

Further, when the exhaust air-fuel ratio becomes stoichiometric or rich,the oxygen concentration of the exhaust flowing-in is extremely reduced,so that NO₂ or NO is released from the main NOx catalyst 20, and thisNO₂ or NO reacts with the unburned HC and CO to be thereby reduced to N₂as shown in FIG. 4(B).

That is, the HC and CO in the exhaust flowing-in immediately react withthe O₂ ⁻ or O²⁻ on the platinum Pt to be thereby oxidized, and even whenthe O₂ ⁻ or O²⁻ on the platinum Pt is consumed, if there still remainssome of the HC and CO, the NOx released from the NOx catalyst and theNOx discharged from the internal combustion engine 1 are reduced to N₂by the HC and CO.

In this way, when NO₂ or NO ceases to exist on the surface of theplatinum Pt, NO₂ or NO is successively released from the main NOxcatalyst 20 and further reduced to N₂. Thus, when the exhaust air-fuelratio is made stoichiometric or rich, NOx is released from the main NOxcatalyst 20 in a short time.

In this way, when the exhaust air-fuel ratio becomes lean, NOx isabsorbed by the main NOx catalyst 20, and when the exhaust air-fuelratio is made stoichiometric or rich, NOx is released from the main NOxcatalyst 20 in a short time to be reduced to N₂. Thus, it is possible toprevent NOx from being discharged to the atmosphere.

As described above, in this embodiment, when the engine operatingcondition is in the full-load operating range, the air-fuel ratio of theair-fuel mixture supplied to the combustion chamber 3 is made rich; whenthe engine operating condition is in the high-load operating range,warming-up operating range after startup, acceleration operating range,and normal operating range at a speed higher than a predetermined speed,the air-fuel ratio of the air-fuel mixture supplied to the combustionchamber 3 is made stoichiometric; and when the engine operatingcondition is in the low-middle load operating range, the air-fuel ratioof the air-fuel mixture is made lean, so that, in the low-middle loadoperating range, the NOx in the exhaust is absorbed by the main NOxcatalyst 20, and in the full-load operating range and high-loadoperating range, the NOx which has been absorbed by the main NOxcatalyst 20 is released and reduced.

When the frequency of full-load operation or high-load operation is low,and the frequency of low-middle load operation is high, with itsoperation time being longer, the amount of NOx absorbed by the main NOxcatalyst 20 is larger than the amount of NOx released from the NOxcatalyst 20 and reduced, so that there is the danger of the NOxabsorbing capacity of the main NOx catalyst 20 being saturated.

In view of this, in this embodiment, when the air-fuel mixture of leanair-fuel ratio is being burnt in the internal combustion engine 1, thatis, when the engine operating condition is in the low-middle loadoperating range, a rich spike control is executed, in which the air-fuelratio of the air-fuel mixture is controlled such that the air-fuelmixture of stoiometric or rich air-fuel ratio is burnt at a relativelyshort period in a spike-like manner (short time), effecting thereleasing and reduction of NOx on a short periodic basis.

At this time, it is preferable that the NOx absorption amount of themain NOx catalyst 20 is monitored, and the rich spike control isexecuted at the time when the NOx absorption amount has reached apredetermined amount (the limit value of NOx amount that can be absorbedby the main NOx catalyst 20). In the following description, the controloperation for the internal combustion engine 1 in which the exhaustair-fuel ratio (the air-fuel ratio of the air-fuel mixture in thisembodiment) alternates at a relatively short period between “leanair-fuel ratio” and “spike-like stoiometric air-fuel ratio or richair-fuel ratio” for the absorption and releasing of NOx by the main NOxcatalyst 20, will be referred to as a lean/rich spike control. In thepresent application, the lean/rich spike control is included in the leanair-fuel ratio control.

On the other hand, sulfur (S) is contained in the fuel, and therefore,when the sulfur in the fuel burns, sulfur oxides (SOx), such as SO₂ andSO₃, are generated, and the main NOx catalyst 20 also absorbs these SOxin the exhaust. It is assumed that the SOx absorption mechanism of themain NOx catalyst 20 is the same as the NOx absorption mechanism. Thatis, as in the case of the description of the NOx absorption mechanism, acase will be described as an example in which platinum Pt and barium Baare carried on the carrier, and as described above, when the exhaustair-fuel ratio is lean, oxygen O₂ adheres to the surface of the platinumPt of the main NOx catalyst 20 in the form of O₂ ⁻ or O²⁻, and the SOx(for example, SO₂) in the exhaust flowing-in is oxidized on the surfaceof the platinum Pt to become SO₃.

Thereafter, the produced SO₃ is absorbed by the main NOx catalyst 20 tobe combined with barium oxide BaO while being further oxidized on thesurface of the platinum Pt, and is diffused into the main NOx catalyst20 in the form of sulfate ions SO₄ ²⁻ to generate sulfate BaSO₄. Thissulfate BaSO₄ is stable and hard to decompose, and is not decomposedeven if the air-fuel ratio of the exhaust flowing-in is made rich, andwill remain in the main NOx catalyst 20. Thus, when, with passage oftime, the amount of BaSO₄ produced in the main NOx catalyst 20increases, the amount of BaO that can contribute to the absorption ofthe main NOx catalyst 20 decreases, resulting in a deterioration in NOxabsorbing capacity. This is what is called SOx poisoning.

In view of this, in this embodiment, in order that no SOx may flow intothe main NOx catalyst 20, there is provided on the upstream side of themain NOx catalyst 20 an SOx absorbing material 17 which absorbs SOx whenthe air-fuel ratio of the exhaust flowing-in is lean, and which releasesthe SOx it has absorbed when the air-fuel ratio of the exhaustflowing-in becomes stoichiometric or rich to cause lowering of oxygenconcentration. This SOx absorbing material 17 absorbs both SOx and NOxwhen the air-fuel ratio of the exhaust flowing into the SOx absorbingmaterial 17 is lean, but when the air-fuel ratio of the exhaustflowing-in becomes stoichiometric or rich to cause lowering of oxygenconcentration, it releases not only SOx but also NOx.

As described above, the stable sulfate BaSO₄ is produced in the main NOxcatalyst 20 when it absorbs SOx. As a result, even when the air-fuelratio of the exhaust flowing into the main NOx catalyst 20 is madestoichiometric or rich, SOx ceases to be released from the main NOxcatalyst 20. Thus, to cause SOx to be released from the SOx absorbingmaterial 17 when the air-fuel ratio of the exhaust flowing into the SOxabsorbing material 17 is made stoichiometric or rich, it is necessaryfor SOx to exist in the SOx absorbing material 17 in the form of sulfateions SO₄ ²⁻, or, even if BaSO₄ is produced, it is necessary for theBaSO₄ to exist in the unstable state in the SOx absorbing material 17.As an SOx absorbing material 17 making this possible, it is possible touse the material comprising the alumina carrier which carries thereon atleast one of the following metals selected from: transition metals, suchas copper Cu, iron Fe, manganese Mn, and nickel Ni; sodium Na; titaniumTi; and lithium Li.

In this SOx absorbing material 17, when the air-fuel ratio of theexhaust flowing into the SOx absorbing material 17 is lean, the SO₂ inthe exhaust is absorbed in the SOx absorbing material 17 in the form ofsulfate ions SO₄ ²⁻ while being oxidized on the surface of the SOxabsorbing material 17, and then diffused into the SOx absorbing material17. In this case, when one of platinum Pt, palladium Pd, and rhodium Rhis carried on the carrier of the SOx absorbing material 17, the SO₂easily adheres to the platinum Pt, palladium Pd, or rhodium Rh in theform of SO₂ ³⁻, thereby the SO₂ is easily absorbed in the SOx absorbingmaterial 17 in the form of sulfate ions SO₄ ²⁻. Thus, to promote theabsorption of SO₂, it is preferable to have one of platinum Pt,palladium Pd, and rhodium Rh carried on the carrier of the SOx absorbingmaterial 17.

When this SOx absorbing material 17 is arranged on the upstream side ofthe main NOx catalyst 20, the SOx in the exhaust is absorbed by the SOxabsorbing material 17 when the air-fuel ratio of the exhaust flowinginto the SOx absorbing material 17 becomes lean, so that no SOx flowsinto the main NOx catalyst 20 on the downstream side, and only the NOxin the exhaust is absorbed by the main NOx catalyst 20.

On the other hand, as described above, the SOx absorbed by the SOxabsorbing material 17 is diffused in the SOx absorbing material 17 inthe form of sulfate ions SO₄ ²⁻ or exists as sulfate BaSO₄ in theunstable state. Thus, when the air-fuel ratio of the exhaust flowinginto the SOx absorbing material 17 becomes stoichiometric or rich tocause lowering of oxygen concentration, the SOx absorbed by the SOxabsorbing material 17 is easily released from the SOx absorbing material17. Further, the SOx absorbing material 17, constructed as describedabove, has a so-called 3-way purifying function which it purifies theHC, CO, and NOx in the exhaust when the air-fuel ratio of the exhaust isclose to the stoichiometric air-fuel ratio.

Further, studies made by the present applicant has shown the followingfact regarding the absorption/releasing action of the SOx absorbingmaterial 17. When the amount of SOx absorbed in the SOx absorbingmaterial 17 is small, the SOx absorbing power of the SOx absorbingmaterial 17 is strong, so that no SOx is released from the SOx absorbingmaterial 17 by causing the exhaust of stoichiometric or rich air-fuelratio to flow into the SOx absorbing material 17 for a short period oftime (for example, less than five seconds). In this regard, the presentapplicant has ascsertained that when the amount of SOx absorbed in theSOx absorbing material 17 is small, no SOx is released from the SOxabsorbing material with the duration time of the stoichiometric or richair-fuel ratio for performing the lean/rich spike control for thepurpose of releasing NOx from the main NOx catalyst 20. However, evenwhen the amount of SOx absorbed in the SOx absorbing material 17 issmall, SOx is released from the SOx absorbing material in the case inwhich the exhaust of stoichiometric or rich air-fuel ratio is caused toflow through the SOx absorbing material 17 for a long period of time.

However, when the amount of SOx absorbed in the SOx absorbing material17 increases, the SOx adsorbing power of the SOx absorbing material 17is weakened, so that SOx is leaked out from the SOx absorbing material17 also when the exhaust of stoichiometric or rich air-fuel ratio iscaused to flow through the SOx absorbing material 17 for a short periodof time, threby there is the danger of the NOx catalyst 20 on thedownstream side undergoing SOx poisoning.

In view of this, in this embodiment, the amount of SOx absorbed in theSOx absorbing material 17 is estimated from the operation history of theinternal combustion engine 1, and the time when the estimated SOxabsorption amount reaches a predetermined amount is determined to be theregeneration time for the SOx absorbing material 17, and regenerationprocessing is executed to cause SOx to be released from the SOxabsorbing material 17.

In executing the regeneration processing for the SOx absorbing material17, the ECU 30 determines the engine operating condition at that timefrom the engine RPM N and the engine load Q/N, and uses the exhausttemperature detected by the temperature sensor 29 as the temperature ofthe SOx absorbing material 17, and on the basis of the engine operatingcondition and the temperature of the SOx absorbing material 17, selectsan air-fuel ratio condition and a processing time allowing efficientreleasing of SOx while suppressing a deterioration in fuel efficiency toa minimum. The ECU 30 causes the exhaust of the selected air-fuel ratiocondition to flow through the SOx absorbing material 17 for the selectedprocessing time, thereby effecting regeneration processing of the SOxabsorbing material 17.

On the other hand, it is known that, to release SOx from the SOxabsorbing material 17, it is necessary for the SOx absorbing material 17to be at a high temperature higher than a predetermined temperature (forexample, 550° C.). During the execution of the regeneration processingof the SOx absorbing material 17, the ECU 30 controls the temperature ofthe exhaust by an appropriate means, and controls the temperature of theSOx absorbing material 17 to be higher than the predeterminedtemperature (hereinafter, this will be referred to as the SOx releasingtemperature).

When the SOx absorbing material 17 is regenerated, the exhaust flowingout from the SOx absorbing material 17 (hereinafter referred to as theregenerated exhaust) contains a large amount of SOx released from theSOx absorbing material 17, so that when this regenerated exhaust flowsinto the main NOx catalyst 20, the SOx in the regenerated exhaust isabsorbed by the main NOx catalyst 20, and the main NOx catalyst 20undergoes SOx poisoning, which makes the provision of the SOx absorbingmaterial 17 meaningless. In view of this, in this embodiment, to preventthe SOx released from the SOx absorbing material 17 at the time ofregeneration of the SOx absorbing material 17 from being absorbed by theNOx catalyst 20, the regenerated exhaust flowing out from the SOxabsorbing material 17 is guided into the bypass pipe 26 at the time ofregeneration of the SOx absorbing material 17.

Next, the flow of exhaust when the SOx absorbing material 17 undergoesregeneration processing and when it undergoes no regeneration processingwill be described.

First, the case in which the SOx absorbing material 17 undergoes noregeneration processing will be described. In this case, the NOx in theexhaust is reduced and purified through absorption and releasing by themain NOx catalyst 20, so that the lean/rich spike control is executed,and the exhaust switching valve 28 is retained at the bypass-closingposition as indicated by the solid line in FIG. 1. Thus, at this time,the exhaust flowing out from the SOx absorbing material 17 flows intothe main NOx catalyst 20. And, the SOx in the exhaust is absorbed by theSOx absorbing material 17, and only the NOx in the exhaust is absorbedand released by the main NOx catalyst 20 to undergo reduction andpurification.

When the SOx absorbing material 17 undergoes no regeneration processing,the exhaust switching valve 28 is retained at the bypass closingposition, so that no exhaust ought to flow to the bypass pipe 26.However, since the sealing property of the exhaust switching valve 28 isnot perfect, some exhaust can leak to the bypass pipe 26 from theexhaust switching valve 28. However, in the exhaust emission purifyingdevice of this embodiment, if the exhaust leaks to the bypass pipe 26,the leaking exhaust flows through the sub NOx catalyst 24 provided inthe bypass pipe 26 at a very low spatial velocity (low SV), so that theHC and NOx in the exhaust are purified by the sub NOx catalyst 24. Thisis due to the fact that the sub NOx catalyst 24 consists of a selectivereduction type NOx catalyst, and the selective reduction type NOxcatalyst exhibits high NOx purifying ratio with a small amount of HC atlow SV, and purifies HC and NOx at a purifying ratio of 70 to 80%.Further, at this time, the sub NOx catalyst 24 absorbs the SOx in theexhaust flowing through it as sulfuric acid.

In this way, if some exhaust leaks to the bypass pipe 26 when the SOxabsorbing material undergoes no regeneration processing, the leakingexhaust is.also purified by the sub NOx catalyst 24, so that no exhaustis discharged to the atmosphere from the vehicle without having beenpurified, thereby achieving an improvement in reliability in exhaustemission purification.

Next, the case in which SOx is to be released from the SOx absorbingmaterial 17, i.e., the case in which the SOx absorbing material 17undergoes regeneration processing will be described. At this time, theair-fuel ratio control of the internal combustion engine 1 is switchedfrom the lean/rich spike control to the stoichiometric control or richair-fuel ratio control. At the same time, the exhaust switching valve 28is switched from the bypass closing position to the bypass openingposition indicated by the broken line in FIG. 1 and retained in thisposition. When the exhaust of stoichiometric or rich air-fuel ratioflows into the SOx absorbing material 17, SOx is released from the SOxabsorbing material 17. However, at this time, the regenerated exhaustflowing out of the SOx absorbing material 17 does not flow into the mainNOx catalyst 20 but flows into the bypass pipe 26. Thus, it is possibleto prevent the main NOx catalyst 20 from undergoing SOx poisoning by theSOx in the regenerated exhaust.

Further, the exhaust of stoichiometric or rich air-fuel ratio flowinginto the bypass pipe 26 passes through the sub NOx catalyst 24, so that,in this process, the SOx which has been absorbed by the sub NOx catalyst24 in the form of sulfuric acid is also released from the sub NOxcatalyst 24. This is because the selective reduction type NOx catalystconstituting the sub NOx catalyst 24 releases SOx as long as theair-fuel ratio of the exhaust flowing-in is stoichiometric or rich evenif the sulfur concentration of the exhaust flowing-in is high.

And, the SOx released from the SOx absorbing material 17 and the sub NOxcatalyst 24 is reduced by the unburned HC and CO in the exhaust andreleased as SO_(2.)

During the regeneration processing of the SOx absorbing material 17, theunburned HC and CO and NOx are discharged from the internal combustionengine 1. However, since the SOx absorbing material 17 and the sub NOxcatalyst 24 have the 3-way purifying function, the unburned HC and COand NOx are purified by the SOx absorbing material 17 and the sub NOxcatalyst 24. Thus, also during the regeneration processing of the SOxabsorbing material 17, there is no danger of unburned HC and CO and NOxbeing released to the atmosphere.

Next, when the regeneration processing of the SOx absorbing material 17is to be stopped, the air-fuel ratio control of the internal combustionengine 1 is switched from the stoichiometric or rich air-fuel ratiocontrol to the lean/rich spike control. At the same time, the exhaustswitching valve 28 is switched from the bypass opening position to thebypass closing position indicated by the solid line in FIG. 1. When theair-fuel ratio of the exhaust becomes the air-fuel ratio correspondingto the lean/rich spike control, the releasing of SOx from the SOxabsorbing material 17 is stopped.

FIG. 5 shows an example of the air-fuel ratio control in thisembodiment. In the lean/rich spike control of this embodiment, when, forexample, the vehicle is running at a normal speed of 60 km/h, 40 secondsof lean air-fuel ratio operation and approximately two seconds ofstoichiometric operation are alternately repeated. On the other hand, atthe time of regeneration processing of the SOx absorbing material 17,the air-fuel ratio is controlled to stoichiometric, and the durationtime thereof is sufficiently longer than the rich spike duration time ofthe lean/rich spike control, for example, approximately one hour.

Next, with reference to FIG. 6, the exhaust flow switching processingexecution routine in this embodiment will be described. A flowchartcomprising steps which constitute this routine is stored in the ROM 32of the ECU 30, and the processing in each step of the flowchart isexecuted by the CPU 34 of the ECU 30.

(Step 101)

First, in Step 101, the ECU 30 determines whether the SOx absorbingmaterial 17 is to be regenerated or not. In the flowchart of FIG. 6, theSOx absorbing material 17 is called “S trap”.

When determined affirmative in Step 101, that is, when it is determinedthat the SOx absorbing material 17 is to be regenerated, the process bythe ECU 30 advances to Step 102, in which it executes the exhausttemperature control such that the temperature of the SOx absorbingmaterial 17 is higher than the SOx releasing temperature, and alwoselects the stoichiometric or rich condition and the regenerationprocessing time that allow most efficient releasing of SOx. In thisembodiment, the temperature control of the exhaust is executed on thebasis of the exhaust temperature at the outlet of the SOx absorbingmaterial 17 detected by the temperature sensor 29.

Next, the process by the ECU 30 advances from Step 102 to Step 103, inwhich it executes the regeneration processing of the SOx absorbingmaterial 17 in accordance with the stoichiometric or rich condition andthe regeneration processing time selected in Step 102. Further, theexhaust switching valve 28 is retained at the bypass opening positionindicated by the broken line in FIG. 1 to introduce the exhaust into thebypass pipe 26, and prevents it from flowing into the NOx catalyst 20.By causing the exhaust of the stoichiometric or rich air-fuel ratio toflow through the SOx absorbing material 17, SOx is released from the SOxabsorbing material 17, and the regenerated exhaust passes through thebypass pipe 26 and the sub NOx catalyst 24 is released to theatmosphere. Thus, no regenerated exhaust flows into the main NOxcatalyst 20, so that SOx poisoning of the main NOx catalyst 20 isprevented. As stated above, also during the regeneration processing ofthe SOx absorbing material 17, the exhaust is purified by the 3-waypurifying function of the SOx absorbing material 17 and the sub NOxcatalyst 24.

After the regeneration processing of the SOx absorbing material 17 hasbeen executed for a predetermined period of time, the process by the ECU30 advances to Step 104, and the regeneration processing of the SOxabsorbing material 17 is completed, and the air-fuel ratio control ofthe internal combustion engine 1 is changed from the stoichiometric orrich air-fuel ratio control to the lean/rich spike control.

Next, the process by the ECU 30 advances to Step 105, in which theexhaust switching valve 28 is switched to the bypass closing positionindicated by the solid line in FIG. 1 to introduce the exhaust into themain NOx catalyst 20, and prevents it from flowing into the bypass pipe26. As a result, the exhaust passes through the SOx absorbing material17 and the main NOx catalyst 20 to be released to the atmosphere. Atthis time, the SOx in the exhaust is absorbed by the SOx absorbingmaterial 17, and only the NOx in the exhaust is absorbed and released bythe main NOx catalyst 20 to be reduced and purified. Further, a smallamount of exhaust leaking from the exhaust switching valve 28 to thebypass pipe 26 is purified when it passes through the sub NOx catalyst24.

Further, also in the case in which the negative determination is made inStep 101, the process by the ECU 30 advances to Step 105, in which theexhaust is caused to flow through the main NOx catalyst 20. After Step105, the process advances to the “return.”

Thus, in this embodiment, at the time of regeneration processing of theSOx absorbing material 17, the exhaust flowing out of the SOx absorbingmaterial 17 flows to the bypass pipe 26, and ceases to flow into themain NOx catalyst 20, so that it is possible to reliably prevent themain NOx catalyst 20 from undergoing SOx poisoning. As a result, it ispossible to constantly maintain the NOx purifying ratio of the main NOxcatalyst 20 at a high level. Further, when the SOx absorbing materialundergoes no regeneration processing, that is, if, when the exhaust isflowing through the main NOx catalyst 20, some exhaust leaks from theexhaust switching valve 28 to the bypass pipe 26, the leaking exhaust ispurified by the sub NOx catalyst 24, so that it does not contaminate theatmosphere. As a result, an improvement is achieved in terms ofreliability in exhaust emission purification.

While in the above-described embodiment the exhaust switching valve 28is controlled at the time of regeneration processing of the SOxabsorbing material 17 so as to cause the exhaust to flow through thebypass pipe 26, if there is the danger of SOx being released from theSOx absorbing material 17 despite the regeneration processing of the SOxabsorbing material 17, the exhaust switching valve 28 may be switched soas to cause the exhaust to flow through the bypass pipe 26.

That is, as stated above, in this internal combustion engine 1, duringthe high-load operation, the warming-up operation after startup, theacceleration operation, and the normal operation at a speed of higherthan 120 km/h, the air-fuel ratio is controlled to be stoichiometric,and during the full-load operation, the air-fuel ratio is controlled tobe rich. Thus, in these operating conditions, the air-fuel ratio of theexhaust is stoichiometric or rich, and the exhaust of stoichiometric orrich air-fuel ratio flows into the SOx absorbing material 17.

If the exhaust of stoichiometric or rich air-fuel ratio flows into theSOx absorbing material 17, no problem is involved as long as the flowingis in an extremely short duration. However, the exhaust flowingcontinues to some degree, there is the danger of SOx being released fromthe SOx absorbing material 17 when the exhaust temperature becomeshigher than the SOx releasing temperature. When this exhaust flows intothe main NOx catalyst 20 on the downstream side, there is the danger ofthe main NOx catalyst 20 undergoing SOx poisoning.

Thus, when the air-fuel ratio of the exhaust has become stoichiometricor rich due to the demand based on the operating condition of theinternal combustion engine 1, such as the high-load operation, thewarming-up operation after startup, the acceleration operation, thenormal operation at a speed of higher than 120 km/h, and the full-loadoperation, it is possible to further reliably prevent SOx poisoning ofthe main NOx catalyst 20 by switching the exhaust switching valve 28 soas to introduce the exhaust into the bypass pipe 26.

Second Embodiment

Next, a second embodiment of the exhaust emission purifying device foran internal combustion engine of the present invention will bedescribed. The difference between the second embodiment and the firstembodiment is as follows.

In the second embodiment, as the sub NOx catalyst 24 provided in thebypass pipe 26, a selective reduction type NOx catalyst having a 3-waypurifying function and HC adsorbing capacity at low temperature is used.The HC adsorbing capacity of the sub NOx catalyst 24 can be enhanced by,for example, increasing the amount of zeolite in the carrier of the subNOx catalyst 24. The reason for providing the sub NOx catalyst 24 havingsuch characteristics is as follows.

In the above-described first embodiment, when the exhaust is introducedinto the bypass pipe 26 when there is the danger of SOx being releasedfrom the SOx absorbing material 17, the SOx absorbing material 17 hasnot attained the SOx releasing temperature even when the air-fuel ratiohas become stoichiometric or rich in the low-temperature start or thelike, so that it is determined that there is no danger of SOx beingreleased from the SOx absorbing material 17, and the exhaust switchingvalve 28 is controlled so as to introduce the exhaust into the main NOxcatalyst 20.

If, under this temperature condition, the exhaust is caused to flowthrough the SOx absorbing material 17, the HC in the exhaust is notpurified and it only flows to the downstream side. Further, since themain NOx catalyst 20 has not attained the activation temperature yet,the exhaust is not purified by causing it to flow through the main NOxcatalyst 20, the HC being allowed to pass along.

Thus, in the second embodiment, when the SOx absorbing material 17 hasnot reached the activation temperature at which HC can be purified, theexhaust switching valve 28 is retained at the bypass opening position soas to introduce the exhaust to the bypass pipe 26, and, by utilizing theHC absorbing capacity of the sub NOx catalyst 24 provided in the bypasspipe 26, the HC in the exhaust is adsorbed by the sub NOx catalyst 24.It has been ascertained by the present applicant that the HC adsorbed bythe sub NOx catalyst 24 is changed to an HC which is highly reactivewith NOx.

And, when the SOx absorbing material 17 has reached a temperature atwhich HC can be purified, the exhaust switching valve 28 is switched tothe bypass closing position so as to introduce the exhaust to the mainNOx catalyst 20. Even when the exhaust switching valve 28 is retained atthe bypass closing position, it can happen that a small amount ofexhaust leaks from the exhaust switching valve 28 to the bypass pipe 26.However, the leaking exhaust flows through the sub NOx catalyst 24 at avery low spatial velocity (low SV), and the HC and NOx in the exhaustare purified by the sub NOx catalyst 24. At the same time, the HCabsorbed by the sub NOx catalyst 24 and made highly reactive with NOxreacts with the NOx in the exhaust, and with the oxygen in the exhaust,whereby it is separated from the sub NOx catalyst and purified.

Next, with reference to FIG. 7, the exhaust flow switching processingexecution routine will be described. A flowchart comprising the stepswhich constitute this routine is stored in the ROM 32 of the ECU 30, andthe processing in each step of the flowchart is all executed by the CPU34 of the ECU 30.

(Step 201)

First, in Step 201, the ECU 30 determines whether the temperature of theSOx absorbing material 17 is less than the temperature at which HC canbe purified (catalyst activation temperature) or not. In thisembodiment, the exhaust temperature at the outlet of the SOx absorbingmaterial 17 detected by the temperature sensor 29 is used as thetemperature of the SOx absorbing material 17. In the flowchart of FIG.7, the SOx absorbing material 17 is called “S trap”.

(Step 202)

When determined affirmative in Step 201, that is, when it is determinedthat the temperature of the SOx absorbing material 17 is less than thetemperature at which HC can be purified, the process by the ECU 30advances to Step 202, in which it retains the exhaust switching valve 28at the bypass opening position so as to introduce the exhaust to thebypass pipe 26, whereby the exhaust flows through the sub NOx catalyst24, and the HC in the exhaust is adsorbed by the sub NOx catalyst 24.

(Step 203)

On the other hand, when determined negative in Step 201, that is, whenit is determined that the temperature of the SOx absorbing material 17has reached the temperature at which HC can be purified, the process ofthe ECU 30 advances to Step 203.

(Steps 203 through 207)

Steps 203 through 207 are completely the same as Steps 101 through 105of the flowchart of the first embodiment shown in FIG. 6, so that adescription thereof will be omitted.

In this way, in the second embodiment, the exhaust is released to theatmosphere after having been purified also when the internal combustionengine is started at a low temperature, so that an improvement isachieved in terms of reliability in exhaust emission purification.

Third Embodiment

Next, a third embodiment of the exhaust emission purifying device for aninternal combustion engine of the present invention will be describedwith reference to FIGS. 8 through 11.

FIG. 8 is a diagram schematically showing the construction of theexhaust emission purifying device of the third embodiment. In thisembodiment, the present invention is applied to an in-cylinder injectiontype vehicle lean-burn gasoline engine capable of lean burning.

In the drawing, numeral 1 indicates a main body of an in-linefour-cylinder internal combustion engine, numeral 2 indicates a piston,numeral 3 indicates a combustion chamber, numeral 4 indicates a sparkingplug, numeral 5 indicates an intake valve, numeral 6 indicates an intakeport, numeral 7 indicates an exhaust valve, numeral 8 indicates anexhaust port, and numeral 11 indicates a fuel injection valve. In thisinternal combustion engine 1, the fuel is directly injected into thecombustion chamber 3 from the fuel injection valve 7.

The intake port 6 is connected to a surge tank 10 through each branchpipe of an intake manifold 9, and the surge tank 10 is connected to anintake duct 12. The intake duct 12 is connected to an airflow meter 13adapted to output a voltage in proportion to the mass of intake air, andthe airflow meter 13 is connected to an air cleaner 14.

Arranged at- some midpoint in the intake duct 12 is a throttle valve 15for adjusting the intake flow rate in the intake duct 12. Mounted tothis throttle valve 15 are a throttle motor 15 a comprising a DC motoror the like for opening and closing the throttle valve 15 in accordancewith the magnitude of the applied voltage and a throttle position sensor15 b for outputting an electric signal corresponding to the openingdegree of the throttle valve 15.

The airflow meter 13 and the throttle position sensor 15 b areelectrically connected through respectively corresponding A/D converters38 to an input port 35 of the ECU 30, and output signals of the sensorsare input into the ECU 30.

On the other hand, regarding the exhaust ports 8, the exhaust ports 8 ofa first cylinder 1A and a fourth cylinder 1D are, as shown in FIG. 9,connected to a casing 50A of a first start converter through a firstexhaust manifold 16A, and the exhaust ports 8 of a second cylinder 1Band a third cylinder 1C are connected to a casing 50B of a second startconverter through a second exhaust manifold 16B. The casings 50A and 50Beach contains a 3-way catalyst 51 having SOx absorbing capacity. Thatis, this 3-way catalyst 51 comprises an ordinary 3-way catalyst carryingthereon an SOx absorbing agent (for example, barium Ba, potassium K, orlanthanum La).

The casings 50A and 50B are connected to an exhaust pipe 53 throughexhaust pipes 52A and 52B, and the exhausts from the cylinders join inthe exhaust pipe 53. The exhaust pipe 53is connected through an exhaustpipe 54 to a casing 56 containing an occlusion reduction type NOxcatalyst 55, and the casing 56 is connected to an exhaust pipe 58through an exhaust pipe 57, the exhaust pipe 58 being connected to anunillussstrated muffler. In the following, this occlusion reduction typeNOx catalyst 55 will be called the main NOx catalyst 55.

Further, the exhaust pipe 53 and the exhaust pipe 58 are also connectedby a bypass path 59 bypassing the main NOx catalyst 55. The bypass path59 is formed by a bypass pipe 59A connected to the exhaust pipe 53, abypass pipe 59B connected to the exhaust pipe 58, and a casing 60 placedbetween the bypass paths 59A and 59B, and the casing 60 contains anocclusion reduction type NOx catalyst 61. In the following, thisocclusion reduction type NOx catalyst 61 will be called the sub NOxcatalyst 61. The construction of the main NOx catalyst 55 and the subNOx catalyst 61 of the third embodiment is completely the same as thatof the main NOx catalyst 20 of the first embodiment, and a descriptionthereof will be omitted.

Provided in the exhaust pipe 54 situated on the upstream side of themain NOx catalyst 55 is a first exhaust switching valve 63 for openingand closing the flow passage of the exhaust pipe 54. Mounted to thisfirst exhaust switching valve 63 is a first actuator 62 for opening andclosing the first exhaust switching valve 63 in accordance with themagnitude of the applied current.

Provided in the bypass pipe 59A situated on the upstream side of the subNOx catalyst 61 is a second exhaust switching valve 65 for opening andclosing the exhaust passage in the bypass pipe 59A. Mounted to thissecond exhaust switching valve 65 is a second actuator 64 for openingand closing the second exhaust switching valve 65 in accordance with themagnitude of the applied current.

Mounted to the exhaust pipe 53 are a temperature sensor 66 forgenerating an output voltage in proportion to the temperature of theexhaust passing through the 3-way catalyst 51, and an oxygenconcentration sensor 67 for generating an output voltage in proportionto the oxygen concentration of the exhaust. Mounted to the exhaust pipe57 is an oxygen concentration sensor 68 for generating an output voltagein proportion to the oxygen concentration of the exhaust passing throughthe main NOx catalyst 55.

Next, the output voltages of the temperature sensor 66 and the oxygenconcentration sensors 67 and 68 are input to the input port 35 of theECU 30 through the corresponding A/D converters 38. Further, input froma RPM sensor 41 to the input port 35 of the ECU 30 is an output pulseindicating the engine RPM.

An output port 36 of the ECU 30 is electrically connected throughcorresponding driving circuits 39 to the sparking plug 4, the fuelinjection valve 11, the throttle motor 15 a, and the first actuator 62and the second actuator 64 of the first and second exhaust switchingvalves 63 and 65.

In the third embodiment, the ECU 30 executes the lean air-fuel ratiocontrol at the engine start, executes the lean air-fuel ratio controlwhen the engined operating condition is in the low-middle load operatingrange, and executes the stoichiometric control when the engine operatingcondition is in the warming-up operating range and when the engineoperating condition is in the acceleration operating range. Regardingthe high-load operating range, the ECU 30 executes the stoichiometriccontrol in the range where the engine load is particularly high andexecutes the lean air-fuel ratio control in the other range. Regardingthe normal operation range, the ECU 30 performs the stoichiometriccontrol in the range where the speed is particularly high, and performsthe lean air-fuel ratio control in the other range.

In the third embodiment, the 3-way catalyst 51 contained in the casings50A and SOB not only functions as a start converter for purifying theexhaust when the exhaust of stoichiometric air-fuel ratio is dischargedin warming-up operation or the like after the engine start, but alsofunctions as the SOx absorbing material 17 described with reference tothe first embodiment. That is, when the exhaust of lean air-fuel ratioflows through the 3-way catalyst 51, the SOx in the exhaust is absorbedby the 3-way catalyst 51, and when the exhaust of stoichiometric or richair-fuel ratio flows through the 3-way catalyst, the SOx absorbed by the3-way catalyst 51 is released.

In the first embodiment, when it is determined that the amount of SOxabsorbed by the SOx absorbing material 17 has reached a predeterminedamount, the ECU 30 positively controls the internal combustion engine 1so as to cause the SOx absorbed by the SOx absorbing material 17 to bereleased. In the third embodiment, in contrast, it does not positivelycontrol the internal combustion engine 1 so as to cause the SOx absorbedby the 3-way catalyst 51 to be released.

In this case, the SOx absorbing/releasing action of the 3-way catalyst51 is effected in accordance with the engine operating condition. Thatis, when the operating condition of the internal combustion engine 1 isin the lean air-fuel ratio control execution range, the SOx in theexhaust is absorbed by the 3-way catalyst 51. When the operatingcondition of the internal combustion engine 1 is in the stoichiometriccontrol execution range or the rich air-fuel ratio control executionrange, the air-fuel ratio of the exhaust is stoichiometric or rich, sothat the SOx absorbed by the 3-way catalyst 51 is released if thetemperature of the 3-way catalyst 51 at that time satisfies the SOxreleasing condition.

In the third embodiment, the ECU 30 controls the first and secondexhaust switching valves 63 and 65 in accordance with theabsorbing/releasing operation of the 3-way catalyst 51, in other words,according to changes in the operating condition of the internalcombustion engine 1.

For example, when the engine operating condition is in the lean air-fuelratio control execution range, the ECU 30 determines that the 3-waycatalyst 51 is in the condition in which it can absorb the SOx in theexhaust, and retains the first exhaust switching valve 63 in the fullyopen state and retains the second exhaust switching valve 65 in thetotally closed state, causing the exhaust to flow through the main NOxcatalyst 55 and allowing no exhaust to flow through the sub NOx catalyst61.

In this case, the SOx in the exhaust discharged from the internalcombustion engine 1 is absorbed by the 3-way catalyst 51, and theexhaust from which SOx has been removed flows through the main NOxcatalyst 55, so that the main NOx catalyst 55 does not undergo SOxpoisoning. And, when the exhaust flows through the main NOx catalyst 55,the NOx in the exhaust is absorbed by the main NOx catalyst 55.

As stated above, in the third embodiment, when the lean air-fuel ratiocontrol is being performed on the internal combustion engine 1, thefirst exhaust switching valve 63 and the second exhaust switching valve65 are controlled such that the entire amount of exhaust discharged fromthe internal combustion engine 1 flows through the main NOx catalyst 55.When the stoichiometric control or rich air-fuel ratio control is beingperformed on the internal combustion engine 1, the first exhaustswitching valve 63 and the second exhaust switching valve 65 arecontrolled such that the entire amount of exhaust discharged from theinternal combustion engine 1 flows through the sub NOx catalyst 61.Thus, no exhaust of stoichiometric or rich air-fuel ratio flows throughthe main NOx catalyst 55, so that it is necessary to appropriatelyrelease and purify the NOx absorbed by the main NOx catalyst 55.

In view of this, in the third embodiment, the ECU 30 performs aso-called lean/rich spike control, in which, when the engine operatingcondition is in the lean air-fuel ratio control execution range, theinternal combustion engine 1 is operated at the lean air-fuel ratio, andthe amount of NOx absorbed by the main NOx catalyst 55 is estimated, andwhen the estimated value reaches a limit value of the NOx amount thatcan be absorbed by the main NOx catalyst 55, the rich spike control isexecuted to cause the NOx absorbed by the main NOx catalyst 55 to bereleased and reduced.

When the lean/rich spike control is being performed, the second exhaustswitching valve 65 is retained in the totally closed state, and noexhaust ought to flow through the bypass path 59. However, since thesealing property of the exhaust switching valve 65 is not perfect, itcan happen that some exhaust leaks from the second exhaust switchingvalve 65 and flows through the bypass path 59.

To cope with this problem, in the exhaust emission purifying device ofthe third embodiment, the sub NOx catalyst 61 is provided in the bypasspath, so that, when the exhaust leaks into the bypass path 59, theleaking exhaust flows through the sub NOx catalyst 61 at a very lowspatial velocity (low SV). Thus, the NOx in the exhaust is purified bythe occlusion reduction type NOx catalyst constituting the sub NOxcatalyst 61.

In this way, when the engine operating condition is in the lean/richspike control execution range, any exhaust allowed to leak into thebypass path 59 is purified by the sub NOx catalyst 61, so that theexhaust is not discharged to the atmosphere without purification of theharmful gas components in the exhaust, thereby achieving an improvementin terms of reliability in exhaust purification.

On the other hand, when the operating condition of the internalcombustion engine 1 is in the stoichiometric control execution range orthe rich air-fuel ratio control execution range, and the temperature ofthe 3-way catalyst 51 satisfies the SOx releasing condition, the ECU 30determines that the SOx absorbed by the 3-way catalyst 51 can bereleased, and retains the first exhaust switching valve 63 in thetotally closed state and the second exhaust switching valve 65 in thefully open state thereby cause the exhaust to flow through the sub NOxcatalyst 61 and allow no exhaust to flow through the main NOx catalyst55.

In this case, the exhaust containing the SOx released from the 3-waycatalyst 51 does not flow into the main NOx catalyst 55, but is led tothe exhaust pipe 58 by way of the bypass path 59, so that the SOxpoisoning of the main NOx catalyst 55 is prevented.

On the other hand, the exhaust containing the SOx released from the3-way catalyst 51 flows through the sub NOx catalyst 61. With theexception of the low-temperature start, the exhaust temperature issufficiently high at the time of acceleration operation, high-speedoperation, and high-load operation, with the exhaust flow rate beinghigh. Further, the occlusion reduction type NOx catalyst constitutingthe sub NOx catalyst 61 is also at a considerably high temperature(which is sometimes higher than the SOx releasing temperature). Thus, inthis condition, even if the SOx concentration in the exhaust is high,SOx is not easily absorbed by the sub NOx catalyst 61. Thus, there islittle possibility of the sub NOx catalyst 61 undergoing SOx poisoning.Further, the harmful gas components contained in the exhaust, such asHC, CO, and NOx, are purified by the 3-way purifying function of the3-way catalyst 51 and the sub NOx catalyst 61.

This control of the first and second exhaust switching valves 63 and 65based on the SOx absorbing/releasing operation of the 3-way catalyst 51will be referred to as the normal exhaust switching control. The normalexhaust switching control is executed in accordance with a normalexhaust switching control routine as shown in FIG. 10. The normalexhaust switching control routine as shown in FIG. 10 is stored inadvance in the ROM 32 of the ECU 30; it is a routine repeated for eachpredetermined time.

(Step 301)

In the normal exhaust switching control routine, the ECU 30 firstdetermines in Step 301 whether the engine operating condition is in thelean/rich spike control execution range or not.

(Step 302)

When determined affirmative in Step 301, the process by the ECU 30advances to Step 302, in which it controls the first actuator 62 so asto retain the first exhaust switching valve 63 in the fully open stateand controls the second actuator 64 so as to retain the second exhaustswitching valve 65 in the totally closed state, thereby causing theexhaust to flow through the main NOx catalyst 55 and allowing no exhaustto flow through the sub NOx catalyst 61.

(Step 303)

When determined negative in Step 301, the process by the ECU 30 advancesto Step 303, in which it controls the first actuator 62 so as to retainthe first exhaust switching valve 63 in the totally closed state andcontrols the second actuator 64 so as to retain the second exhaustswitching valve 65 in the fully open state, thereby causing the exhaustto flow through the sub NOx catalyst 61 and allowing no exhaust to flowthrough the main NOx catalyst 55.

On the other hand, while the first and second exhaust switching valves63 and 65 are basically opened and closed in accordance with theabove-described normal exhaust switching control routine, they arecontrolled in accordance with a control routine different from thenormal exhaust switching control routine only in the following cases:(1) when the engine starts; and (2) when the exhaust temperature ishigh. These two cases will be individually described.

(1) When the Engine Starts

When the internal combustion engine 1 is started at a low temperature,to achieve early activation of the 3-way catalyst 51, the ECU 30executes the following 3-way catalyst temperature raising control. Thatis, in addition to the fuel injection (main injection) for obtaining theengine output, the ECU 30 performs an expansion stroke sub injection inwhich the fuel is secondarily injected during the expansion stroke.Further, it places the first and second exhaust switching valves 63 and65 substantially in the totally closed state to throttle the exhaustflow rate. In the process, the main injection is performed at the leanair-fuel ratio control.

In this case, since the main injection is performed at the lean air-fuelratio control, the oxygen in the exhaust becomes excessive. Further, thefirst and second exhaust switching valves 63 and 65 are placedsubstantially in the totally closed state to throttle the exhaust flowrate, whereby the back pressure increases and the exhaust temperaturerises. If, in this condition, the expansion stroke sub injection iseffected, the sub injection fuel is easily burnt. As a result, theexhaust temperature rapidly increases, and it becomes possible to raisethe temperature of the 3-way catalyst 51 in a short time, making itpossible to effect early activation of the 3-way catalyst 51.

After the activation of the 3-way catalyst 51, the ECU 30 terminates theexecution of the 3-way catalyst temperature raising control. And, theECU 30 starts the execution of the above-described normal exhaustswitching control, and starts the execution of the normal air-fuel ratiocontrol.

(2) When the Exhaust Temperature is High

When the exhaust temperature is high and the exhaust flow rate is highas in the case in which the vehicle speed is high and in which theengine load is high, it is to be expected that the temperature of themain NOx catalyst 55 and the sub NOx catalyst 61 will becomeunnecessarily high.

Here, the occlusion reduction type NOx catalyst constituting the mainNOx catalyst 55 and the sub NOx catalyst 61 is activated when the floortemperature or the atmosphere temperature of the occlusion reductiontype NOx catalyst is in a predetermined catalyst purifying wind range(for example, 250 to 500° C.), making it possible to efficiently purifythe NOx in the exhaust. Thus, when the quantity of heat of the exhaustincreases as stated above, it is assumed that the temperature of themain NOx catalyst 55 and the sub NOx catalyst 61 will become higher thanthe catalyst purifying wind.

In this case, when the operating condition of the internal combustionengine 1 is in the lean/rich spike control execution range, it isassumed that the NOx in the exhaust will not be sufficiently purified bythe main NOx catalyst 55 and the sub NOx catalyst 61, resulting in adeterioration in exhaust emission purification.

In view of this, when the temperature of the exhaust flowing into themain NOx catalyst 55 becomes higher than a pre-set upper limit value(for example, the upper limit value of the catalyst purifying wind) whenthe internal combustion engine 1 is being operated at the lean air-fuelratio, the ECU 30 executes the NOx catalyst temperature rise restrainingcontrol as described below. That is, the ECU 30 controls the firstactuator 62 and the second actuator 64 so as to fully open the first andsecond exhaust switching valves 63 and 65, causing the exhaust to flowthrough both the main NOx catalyst 55 and the sub NOx catalyst 61.

In this case, the amount of exhaust flowing through the main NOxcatalyst 55 is reduced by half as compared with the case in which theexhaust from the internal combustion engine 1 is caused to flow solelythrough the main NOx catalyst 55, so that the quantity of heat the mainNOx catalyst 55 receives from the exhaust is also reduced by half, andthe catalyst temperature of the main NOx catalyst 55 is kept within thecatalyst purifying wind. Similarly, the amount of exhaust flowingthrough the sub NOx catalyst 61 is substantially the same as the amountof exhaust flowing through the main NOx catalyst 55, so that thetemperature of the sub NOx catalyst 61 does not rise excessively and iskept within the catalyst purifying wind.

Thus, in the above-described NOx catalyst temperature rise restrainingcontrol, the exhaust is purified by the main and sub NOx catalysts 55and 61, so that the NOx purifying ratio is substantially improved ascompared with the case in which the entire amount of exhaust is causedto flow through the main NOx catalyst 55.

Further, when the exhaust from the internal combustion engine 1 iscaused to flow through both the main NOx catalyst 55 and the sub NOxcatalyst 61, the spatial velocity of the exhaust is lowered with thereduction in the amount of exhaust flowing through the NOx catalysts 55and 61, so that the NOx purifying ratio of the main and sub NOxcatalysts is further improved.

Regarding the determination of the execution condition of the NOxcatalyst temperature rise restraining control, it is possible, insteadof performing it on the basis of the exhaust temperature, to provide atemperature sensor for detecting the catalyst temperature of the mainNOx catalyst 55, making a determination on the basis of the detectionvalue of this temperature sensor. Further, since the exhaust temperaturecan be estimated from the operating condition of the internal combustionengine 1, it is possible to make a determination according to whetherthe internal combustion engine 1 is in the predetermined operatingcondition or not. In the third embodiment, examples of the predeterminedoperating condition include the lean high-speed operating range and thelean high-load operating range.

The 3-way catalyst temperature rise control and the NOx catalysttemperature rise restraining control are realized through the executionof the catalyst temperature control routine, as shown in FIG. 11, by theECU 30. The catalyst temperature control routine as shown in FIG. 11 isstored in advance in the ROM 32 of the ECU 30, and is a routinerepeatedly executed for each predetermined time.

(Step 401)

In the catalyst temperature control routine, the ECU 30 first determinesin Step 401 whether the internal combustion engine 1 is in the startupcondition or not. Examples of the method for determining the startupcondition of the internal combustion engine 1 includes a method ofdetermining whether the starter switch is ON, whether the engine RPM isnot higher than the predetermined speed, etc.

(Step 402)

When determined affimative in Step 401, the process by the ECU 30advances to Step 402, in which it executes the 3-way catalysttemperature rise processing so as to achieve early activation of the3-way catalyst 51. That is, the ECU 30 controls the first actuator 62and the second actuator 64 so as to make both the first and secondexhaust switching valves 63 and 65 totally closed, executes the leanair-fuel ratio control of the main injection, and further executes theexpansion stroke secondary injection.

(Step 403)

In Step 403, the ECU 30 determines whether the 3-way catalyst 51 hasbeen activated or not. The activation determination of the 3-waycatalyst 51 may, for example, be made as follows. When the exhausttemperature on the downstream side of the 3-way catalyst 51 as detectedby the temperature sensor 66 has attained a predetermined temperature,the catalyst has been activated. When the predetermined temperature hasnot been reached yet, it is determined that the catalyst is notactivated.

It is also possible to provide a catalyst temperature sensor fordirectly detecting the catalyst temperature of the 3-way catalyst 51,making the determination according to whether the catalyst temperaturedetected by the catalyst temperature sensor has reached the activationtemperature or not.

When determined negative in Step 403, the process by the ECU 30 returnsto Step 402, in which the execution of the 3-way catalyst temperaturerise processing is continued. On the other hand, when determinedaffirmative in Step 403, the process by the ECU 30 advances to Step 404.

(Step 404)

In Step 404, the ECU 30 completes the execution of the 3-way catalysttemperature rise processing. After the execution of the processing ofStep 404, the ECU 30 temporarily terminates the execution of thisroutine.

(Step 405)

On the other hand, when determined negataive in Step 401, the process bythe ECU 30 advances to Step 405, in which it determines whether thelean/rich spike air-fuel ratio control is being executed or not.

(Step 406)

When determined affirmative in Step 405, the process by the ECU 30advances to Step 406, in which it determines whether an output signalvalue (exhaust temperature) of the temperature sensor 66 is higher thanthe pre-set upper limit value or not.

(Step 407)

When determined affirmative in Step 406, the process by the ECU 30advances to Step 407, in which the NOx catalyst temperature riserestraining processing is executed. That is, the ECU 30 controls thefirst actuator 62 and the second actuator 64 so as to retain the firstand second exhaust switching valves 63 and 65 in the fully open state,and causes the exhaust to flow through the main NOx catalyst 55 and thesub NOx catalyst 61, purifying the exhaust by the main and sub NOxcatalysts 55 and 61. After finishing the execution of the processing ofStep 407, the ECU 30 temporarily terminates the execution of theroutine.

When determined negative in Step 405, and when determined negative inStep 406, the process by the ECU 30 advances to Step 408.

(Step 408)

In Step 408, the ECU 30 controls the first and second exhaust switchingvalves 63 and 65 in accordance with the above-described normal exhaustswitching control routine. After finishing the execution of theprocessing of Step 408, the ECU 30 temporarily terminates the executionof the routine.

In the third embodiment described above, if some exhaust leaks from thesecond exhaust switching valve 65 to the bypass pipe 60 when the engineoperating condition is in the lean/rich spike control execution range,and the second exhaust switching valve 65 is totally closed so as tocause the exhaust to flow through the main NOx catalyst 55, the leakingexhaust is purified by the sub NOx catalyst 61, so that there is nodeterioration in exhaust emission purification, thereby achieving animprovement in reliability in exhaust purifying.

Further, in the third embodiment, when the catalyst 51 is not activatedyet as in the case in which the internal combustion engine 1 is startedat the low temperature, it is possible to rapidly raise the exhausttemperature by executing the 3-way catalyst temperature rise control,whereby it is possible to achieve an early temperature rise of the 3-waycatalyst 51 to the activation temperature range.

Further, in the third embodiment, when the temperature of the exhaustflowing into the main NOx catalyst 55 has become higher than the pre-setupper limit value, the amount of exhaust flowing through the main NOxcatalyst 55 is reduced by half by executing the NOx catalyst temperaturerise restraining control to thereby reduce by half the quantity of heatthe main NOx catalyst 55 receives from the exhaust and restrain anexcessive temperature rise in the main NOx catalyst 55, making itpossible to retain the temperature of the main NOx catalyst 55 withinthe catalyst purifying wind.

In this process, the amount of exhaust flowing through the sub NOxcatalyst 61 is substantially the same as the amount of exhaust flowingthrough the main NOx catalyst 55, so that the excessive temperature risein the sub NOx catalyst 61 is restrained, making it possible to retainthe temperature of the sub NOx catalyst 61 within the catalyst purifyingwind.

As a result, the exhaust is purified by both the main and sub NOxcatalysts 55 and 61 within the catalyst purifying wind, making itpossible to substantially improve the NOx purifying ratio as comparedwith the case in which the entire amount of exhaust is caused to flowthrough the main NOx catalyst 55.

Further, by causing the exhaust to flow through both the main NOxcatalyst 55 and the sub NOx catalyst 61, it is possible to reduce thespatial velocity of the exhaust in each NOx catalyst, making it possibleto further improve the NOx purifying ratio of each NOx catalyst.

Fourth Embodiment

Next, a fourth embodiment of the exhaust emission purifying device foran internal combustion engine of the present invention will bedescribed. Here, the construction different from that of the thirdembodiment will be described, and a description of the constructionsimilar to that of the third embodiment will be omitted.

The fourth embodiment differs from the above-described third embodimentin that, in the fourth embodiment, the temperature rise control of themain NOx catalyst 55 is performed in addition to the 3-way catalysttemperature rise control and the NOx catalyst temperature riserestraining control described with reference to the third embodiment.

Here, when the engine operating condition is in the warming-up operatingrange after startup, the internal combustion engine 1 is operated at thestoichiometric air-fuel ratio so as to stabilize the combustion state ofthe internal combustion engine 1, and the first exhaust switching valve63 is retained in the totally closed state so as to prevent the SOxreleased from the 3-way catalyst 51 from flowing into the main NOxcatalyst 55, and the second exhaust switching valve 65 is retained inthe fully open state.

In this case, the exhaust discharged from the internal combustion engine1 passes through the sub NOx catalyst 61, and the activation of the subNOx catalyst 61 is achieved along with the warming up of the internalcombustion engine 1.

When the warming up of the internal combustion engine 1 and theactivation of the sub NOx catalyst 61 are completed, the operatingcondition of the internal combustion engine 1 is switched from thestoichiometric operation to the lean/rich spike operation, and the firstexhaust switching valve 63 is switched from the totally closed state tothe fully open state, and the second exhaust switching valve 65 isswitched to the totally closed state from the fully open state, so thatthe exhaust discharged from the internal combustion engine 1 passesthrough the main NOx catalyst 61.

In this process, the exhaust flows through the main NOx catalyst 61 forthe first time after the engine start, so that it is assumed that thetemperature of the main NOx catalyst 61 has not been raised to thecatalyst purifying wind range.

When, in such a case, the exhaust of lean air-fuel ratio is dischargedfrom the internal combustion engine 1, the HC and CO in the exhaust canbe purified to some degree by the 3-way catalyst 51. However, the NOx inthe exhaust cannot be sufficiently purified by the 3-way catalyst 51,and sufficient purification cannot be effected in the main NOx catalyst61, either.

In view of this, in the fourth embodiment, the warming-up operatingrange of the internal combustion engine 1 is extended to the operatingcondition in which the NOx exhaust amount of the internal combustionengine 1 is reduced, preferably, to the operating condition in which theNOx exhaust amount is reduced to zero, after the completion of thewarming up of the internal combustion engine 1 and the activation of thesub NOx catalyst 61.

That is, in the fourth embodiment, the stoichiometric air-fuel ratiooperation of the internal combustion engine 1, the totally closed stateof the first exhaust switching valve 63, and the fully open state of thesecond exhaust switching valve 65 are continued until the operatingcondition is attained in which the NOx exhaust amount of the internalcombustion engine 1 is reduced, after the completion of the warming upof the internal combustion engine 1 and the activation of the sub NOxcatalyst 61.

When the warming up of the internal combustion engine 1 and theactivation of the sub NOx catalyst 61 have been completed and theinternal combustion engine 1 has attained the condition in which the NOxexhaust amount is reduced, the operating condition of the internalcombustion engine 1 is switched from the stoichiometric operation to thelean/rich spike operation, and the first exhaust switching valve 63 isswitched from the totally closed state to the fully open state, and thesecond exhaust switching valve 65 is switched from the fully open stateto the totally closed state.

The engine is in the operating condition in which the NOx exhaust amountis reduced when, for example, the vehicle is running at a deceleratedspeed, when the execution of the fuel injection control is inhibited,and when the execution of the sparking control is inhibited. In thisfourth embodiment, the case will be described in which the vehicle isrunning at a deceleerated speed, as an example.

When the vehicle is in the decelerated running condition, the fuelinjection amount of the internal combustion engine 1 is reduced, or theexecution of the fuel injection is stopped (fuel cut), so that theamount of NOx generated is very small. Further, the exhaust discharagedfrom the internal combustion engine 1 when the vehicle is running at thedecelerated speed receives the heat in the internal combustion engine 1(for example, from the wall surfaces of the intake port, the combustionchamber 3, and the exhaust port) to become a gas with its temperaturehas been increased to some degree even when no combustion is beingperformed in the internal combustion engine 1.

Thus, when the exhaust as described above flows into the main NOxcatalyst 55, there is no substantial deterioration in emission even ifthe main NOx catalyst 55 is not activated yet. Further, the main NOxcatalyst 55 receives the heat from the exhaust to undergo temperaturerise. That is, in the above-described main NOx catalyst temperature risecontrol, it is possible to activate the main NOx catalyst 55 whilerestraining the deterioration in the exhaust emission.

Examples of the method for determining the warming-up completion of theinternal combustion engine 1 include a method of determining whether thetemperature of the cooling water for the engine cooling is higher than apredetermined temperature or not. Examples of the method for determiningthe activation completion of the sub NOx catalyst 61 include a method ofassuming the temperature from the operation history of the internalcombustion engine 1 from the engine start (operating time, theintegrated value of fuel injection amount, the integrated value ofintake air amount, etc.), and a method providing a temperature sensorfor directly detecting the catalyst floor temperature of the sub NOxcatalyst 61 and deetermining whether the detection value of thetemperature sensor is within the range of the catalyst purifying wind ornot.

In the following, the main NOx catalyst temperature rise control of thefourth embodiment will be described with reference to the flowchart ofFIG. 12. The flowchart of FIG. 12 shows the main NOx catalysttemperature rise control routine. This main NOx catalyst temperaturerise control routine is stored in advance in the ROM 32 of the ECU 30;it is a routine executed by the CPU 34 using the start completion of theinternal combustion engine 1 as a trigger.

(Step 501)

In the main NOx catalyst temperature rise control routine, the ECU 30first determines in Step 501 whether the starting of the internalcombustion engine 1 has been completed or not.

When determined negative in Step 501, the ECU 30 executes the processingof Step 501 again. On the other hand, when determined affirmative inStep 501, the process by the ECU 30 advances to Step 502.

(Step 502)

In Step 502, the ECU 30 executes the warming up of the internalcombustion engine 1. More specifically, the ECU 30 operates the internalcombustion engine 1 at the stoichiometric air-fuel ratio, and controlsthe first actuator 62 and the second actuator 64 so as to retain thefirst exhaust switching valve 63 in the totally closed state, and thesecond exhaust switching valve 65 in the fully open state.

In this case, the exhaust of stoichiometric air-fuel ratio is dischargedfrom the internal combustion engine 1, and the harmful gas componentscontained in the exhaust, such as HC, CO, and NOx, are purified by the3-way catalyst 51 activated by the 3-way catalyst temperature risecontrol at the time of engine starting. The exhaust having the harmfulgas components purified by the 3-way catalyst 51 passes through the subNOx catalyst 61 in the bypass path 59 to be led to the exhaust pipe 58.In the process, the sub NOx catalyst 61 is not activated yet. However,as stated above, the harmful gas components in the exhaust have alreadybeen purified by the 3-way catalyst 51, so that there is nodeterioration in exhaust emission. Further, the sub NOx catalyst 61receives the heat from the exhaust to undergo temperatur rise.

(Step 503)

In Step 503, the ECU 30 determines whether the warming up of theinternal combustion engine 1 (and the activation of the sub NOx catalyst61) has been completed or not, by using the temperature of the enginecooling water, the operation history of the internal combustion engine 1from the start, etc. as parameters.

When determined negative in Step 503, the process by the ECU 30 returnsto Step 502, in which the execution of the warming up processing iscontinued. On the other hand, when determined affirmative in Step 503,the process by the ECU 30 advances to Step 504.

(Step 504)

In Step 504, the ECU 30 determines whether the vehicle is running at adecelleerated speed or not. Examples of the method of determining thedecelerated speed running of the vehicle include a method of determiningthat the vehicle is running at the decelerated speed when the operatingamount of an unilliustrated accelerator is “zero” and the vehicle speedis higher than a predetermined speed.

When determined negative in Step 504, the process by the ECU 30 returnsto Step 502, in which the execution of the warming up processing iscontinued. On the other hand, when determined affirmative in Step 504,the process by the ECU 30 advances to Step 505.

(Step 505)

In Step 505, the ECU 30 terminates the execution of the warming upprocessing. More specifically, the ECU 30 switches the operatingcondition of the internal combustion engine 1 from the stoichiometricoperation to the lean/rich spike operation, and controls the firstactuator 62 and the second actuator 64 so as to switch the first exhaustswitching valve 63 from the totally closed state to the fully openstate, and switch the second exhaust switching valve 65 from the fullyopen state to the totally closed state.

In this process, the exhaust discharged from the internal combustionengine 1 flows out to the exhaust pipe 58 by way of the main NOxcatalyst 55. However, the amount of NOx contained in the exhaustdischarged from the internal combustion engine 1 during the deceleratedspeed running is very small, so that even if the main NOx catalyst 55 isnot activated yet, there is no rapid deterioration in exhaust emission.

Further, the exhaust emitted from the internal combustion engine 1during the decelerated speed running of the vehicle receives the heatfrom the interior of the internal combustion engine 1 even when nocombustion is being effected in the internal combustion engine 1, sothat when such exhaust passes through the main NOx catalyst 55, the mainNOx catalyst receives the heat from the exhaust to undergo temperaturerise.

Thus, in the exhaust emission purifying device of the fourth embodiment,in addition to the effect of the third embodiment, it is advantageouslypossible to activate the main NOx catalyst 55 while restrainingdeterioration in exhaust emission.

Fifth Embodiment

Next, a fifth embodiment of the exhaust emission purifying device for aninternal combustion engine of the present invention will be described.Here, the construction different from that of the third embodiment willbe described, and a description of the same construction will beomitted.

The fifth embodiment differs from the third embodiment in that, in thefifth embodiment, SOx poisoning regeneration control of the main NOxcatalyst 55 and the sub NOx catalyst 61 is conducted in addition to the3-way catalyst temperature rise control and the NOx catalyst temperaturerise restraining control.

Here, in the third embodiment described above, the internal combustionengine 1 is not controlled so as to forcibly release the SOx absorbed bythe 3-way catalyst 51, so that, depending upon the operating conditionof the engine, the SOx absorbing capacity of the 3-way catalyst 51 issaturated, and the SOx in the exhaust enters the main NOx catalyst 55without being removed by the 3-way catalyst 51, with the result that themain NOx catalyst 55 undergoes SOx poisoning.

Further, when the internal combustion engine 1 is in the stoichiometricoperating condition or the rich air-fuel ratio operating condition, theSOx that has been absorbed by the 3-way catalyst 51 is released, and thefirst exhaust switching valve 63 is retained in the totally closedstate, and the second exhaust switching valve 65 is retained in thefully open state, so that the SOx released from the 3-way catalyst 51flows through the sub NOx catalyst 61 along with the exhaust. If in thisprocess the temperature of the sub NOx catalyst 61 is not sufficientlyhigh, it may happen that the SOx in the exhaust is absorbed by the subNOx catalyst 61, causing SOx poisoning in the sub NOx catalyst 61.

In view of this, in the fifth embodiment, the degree of SOx poisoning ofthe main NOx catalyst 55 and the sub NOx catalyst 61 is determined, and,on the basis of the determination results, the SOx poisoningregeneration control of the main NOx catalyst 55 and the sub NOxcatalyst 61 is performed.

To eliminate SOx poisoning, it is necessary to raise the temperature ofthe main and sub NOx catalysts 55 and 61 to a high temperature range(for example, 500° C. to 700° C.), and to make the exhaust air-fuelratio in the main and sub NOx catalysts 55 and 61 stoichiometric orrich. Thus, if the SOx poisoning regeneration control is individuallyperformed according to the degree of SOx poisoning in each of the mainNOx catalyst 55 and the sub NOx catalyst 61, the execution frequency ofthe SOx poisoning regeneration control will increase, resulting in anincrease in fuel consumption amount, etc. In view of this, in this fifthembodiment, the regeneration control is simultaneously performed on themain and sub NOx catalysts 55 and 61 when the degree of SOx poisoning ofone of the main NOx catalyst 55 and the sub NOx catalyst 61 exceeds apredetermined standard.

In the following, the SOx poisoning regeneration control of the fifthembodiment will be described with reference to the flowchart of FIG. 13.The flowchart of FIG. 13 shows an SOx poisoning regeneration controlroutine. This SOx poisoning regeneration control routine is stored inadvance in the ROM 32 of the ECU 30; it is a routine repeatedly executedby the CPU 34 for each predetermined time.

(Step 601)

In the SOx poisoning regeneration control routine, the ECU 30 firstexecutes in Step 601 a SOx poisoning degree determination on the mainNOx catalyst 55. Examples of the method for determining the degree ofSOx poisoning of the main NOx catalyst 55 include one in which theair-fuel ratio of the exhaust flowing into the main NOx catalyst 55 ismade lean and then switched to rich air-fuel ratio; a determination ismade on the basis of the time it takes for the output signal value ofthe oxygen sensor (or air-fuel ratio sensor) 68 arranged on thedownstream side of the main NOx catalyst 55 to be reversed from a valueindicating lean air-fuel ratio to a value indicating rich air-fuelratio, that is, the so-called rich reversal time.

(Step 602)

In Step 602, the ECU 30 determines whether the degree of SOx poisoningof the main NOx catalyst 55 determined in Step 601 is higher than apredetermined reference value. The reference value is a value obtainedin advance by experiment, and is stored in the ROM 32, etc. of the ECU30.

When determined negative Step 602, the ECU 30 determines that there isno need to perform SOx poisoning regeneration control on the main NOxcatalyst 55, and the process advances to Step 603.

(Step 603)

In Step 603, the ECU 30 executes the SOx poisoning degree determiningprocessing on the sub NOx catalyst 61. Examples of the method ofdetermining the SOx poisoning degree of the sub NOx catalyst 61 includeone in which estimation is made from the operation history of theinternal combustion engine 1 or the like.

(Step 604)

In Step 604, the ECU 30 determines whether the degree of SOx poisoningof the sub NOx catalyst 61 determined in Step 603 is over apredetermined reference value or not. The reference value is a valueobtained in advance by experiment, and is stored in the ROM 32, etc. ofthe ECU 30.

When determined negative in Step 604, the ECU 30 determines that thereis no need to perform the SOx poisoning regeneration control on the subNOx catalyst 61, and temporarily terminates the execution of theroutine.

On the other hand, when determined affirmative in Step 602 or Step 604,the process by the ECU 30 advances to Step 605.

(Step 605)

In Step 605, the ECU 30 executes the SOx poisoning regenerationprocessing on the main and sub NOx catalysts 55 and 61. Morespecifically, it controls the first actuator 62 so as to retain thefirst exhaust switching valve 63 in the fully open state, controls thesecond actuator 64 so as to retain the second exhaust switching valve 65in the fully open state, and executes NOx catalyst temperature raisingprocessing to raise the temperature of the main and sub NOx catalysts 55and 61 to a predetermined temperature range (500° C. to 700° C.).

Examples of the method of NOx catalyst temperature raising processinginclude the following methods: (1) The internal combustion engine 1 isoperated at rich air-fuel ratio so as to make the air-fuel ratio of theexhaust rich, and a secondary air is supplied to the portion of theexhaust in the exhaust passage on the upstream side of the main and subNOx catalysts 55 and 61, whereby a sufficient amount of unburned fuelcomponents and oxygen is supplied to the main and sub NOx catalysts 55and 61. The unburned fuel components and oxygen are caused to react(burn) in the main and sub NOx catalysts 55 and 61, thereby rapidlyraising the temperature of the main and sub NOx catalysts 55 and 61. (2)The internal combustion engine 1 is operated at stoichiometric or richair-fuel ratio and the main and sub NOx catalysts 55 and 61 are heatedby a dedicated heater. (3) A part of the cylinders of the internalcombustion engine 1 is operated at rich air-fuel ratio, and the rest ofthe cylinders is operated at lean air-fuel ratio, whereby a gas air-fuelmixture of an exhaust containing a sufficient amount of unburned fuelcomponents and an exhaust containing a sufficient amount of oxygen issupplied to the main and sub NOx catalysts 55 and 61; the unburned fuelcomponents and oxygen contained in the gas air-fuel mixture are causedto undergo oxidization in the main and sub NOx catalysts 55 and 61,thereby rapidly raising the temperature of the main and sub NOxcatalysts 55 and 61.

When, in this hardware construction of the fifth embodiment, the abovemethod (3) is adopted, it is desirable that the air-fuel ratio of theexhaust flowing into one of the two 3-way catalysts 51 be rich and thatthe air-fuel ratio of the exhaust flowing into the other be lean.

This is because when the exhaust from a cylinder operated at leanair-fuel ratio and the exhaust from a cylinder operated at rich air-fuelratio flow into the same 3-way catalyst 51, the air-fuel ratio in 3-waycatalyst 51 becomes substantially stoichiometric to cause the unburnedfuel components and oxygen in the exhaust to undergo oxidization in the3-way catalyst 51, making it impossible to cause them to undergooxidization in the main and sub NOx catalysts 55 and 61.

When the above-described SOx poisoning regeneration processing isexecuted, the exhaust from the internal combustion engine 1 flowsthrough both the main NOx catalyst 55 and the sub NOx catalyst 61, andthe spatial velocity of the exhaust flowing through both the main andsub NOx catalysts 55 and 61 is less than that in the case in which theexhaust flows through only one of the main NOx catalyst 55 and the subNOx catalyst 61, so that the SOx poisoning regeneration efficiencyimproves, thereby making it possible to reduce the execution time of theSOx poisoning regeneration control.

As a result, it is possible to reduce the fuel consumption amountrelated to the SOx poisoning regeneration control, and it is possible toreduce the time during which the main and sub NOx catalysts 55 and 61are exposed to high temperature.

(Step 606)

In Step 606, the ECU 30 determines whether the SOx poisoningregeneration of the main and sub NOx catalysts 55 and 61 has beencompleted or not, in other words, whether the SOx poisoning of the mainand sub NOx catalysts 55 and 61 has been eliminated or not.

Examples of the method for determining the SOx poisoning regenerationcompletion include the following methods: a method in which therelationship between the degree of SOx poisoning of the main and sub NOxcatalysts 55 and 61 and the required time for the SOx poisoningregeneration (SOx poisoning regeneration time) is obtained in advance byexperiment, and it is determined that the SOx poisoning of the main andsub NOx catalysts 55 and 61 has been eliminated when the execution timeof the SOx poisoning regeneration control exceeds the SOx poisoningregeneration time; and a method in which SOx sensors adapted to outputelectric signals corresponding to the SOx concentration in the exhaustare arranged, in the exhaust pipe 57 on the downstream side of the mainNOx catalyst 55 and in the bypass path 59 on the downstream side of thesub NOx catalyst 61, and it is determined that the SOx poisoning of themain and sub NOx catalysts 55 and 61 has been eliminated when the outputsignal values of the SOx sensors have become smaller than apredetermined value.

When determined negative in Step 606, the process by the ECU 30 returnsto Step 605, and it continues the execution of the SOx poisoningregeneration processing. On the other hand, when determined affirmativein Step 606, the process by the ECU 30 advances to Step 607.

(Step 607)

In Step 607, the ECU 30 terminates the execution of the SOx poisoningregeneration processing, and returns the control of the internalcombustion engine 1 and the control of the first and second exhaustswitching valves 63 and 65 to normal. After the execution of theprocessing of Step 607, the ECU 30 temporarily terminates the executionof this routine.

In the fifth embodiment described above, when the degree of SOxpoisoning of one of the main NOx catalyst 55 and the sub NOx catalyst 61exceeds a predetermined reference value, the SOx poisoning regenerationis performed simultaneously on the main and sub NOx catalysts 55 and 61,so that as compared with the case in which the main NOx catalyst 55 andthe sub NOx catalyst 61 are individually reactivated, it is possible toreduce the execution frequency of the SOx poisoning regenerationcontrol.

Further, in the fifth embodiment, to simultaneously perform the SOxpoisoning regeneration control on the main and sub NOx catalysts 55 and61, the exhaust from the internal combustion engine 1 is caused to flowthrough both the main NOx catalyst 55 and the sub NOx catalyst 61, sothat, as compared with the case in which the exhaust is caused to flowthrough only one of the main NOx catalyst 55 and the sub NOx catalyst61, the spatial velocity of the exhaust in the NOx catalysts 55 and 61is reduced, thereby improving the SOx regeneration efficiency and makingit possible to reduce the execution time of the SOx poisoningregeneration control.

Thus, in the fifth embodiment, it is possible to reduce the executionfrequency of the SOx poisoning regeneration control of the main and subNOx catalysts 55 and 61, and it is possible to reduce the execution timeof the SOx poisoning regeneration control, so that it is possible toreduce the fuel consumption amount related to the SOx poisoningregeneration control, and to restrain thermal deterioration of the mainand sub NOx catalysts 55 and 61.

Sixth Embodiment

Next, a sixth embodiment of the exhaust emission purifying device for aninternal combustion engine of the present invention will be described.Here, the construction differing from that of the fifth embodiment willbe described, and a description of the construction which is the same asthat of the fifth embodiment will be omitted.

The sixth embodiment differs from the fifth embodiment in that, while inthe fifth embodiment the SOx poisoning regeneration control of the mainNOx catalyst 55 and the the SOx poisoning regeneration control of thesub NOx catalyst 61 are started simultaneously and terminatedsimultaneously, in this sixth embodiment, while the SOx poisoningregeneration control of the main NOx catalyst 55 and the SOx poisoningof the sub NOx catalyst 61 are simultaneously started as in the fifthembodiment, the SOx poisoning regeneration control of the main NOxcatalyst 55 is terminated when the SOx poisoning of the main NOxcatalyst 55 has been eliminated, and the SOx poisoning regenerationcontrol of the sub NOx catalyst 61 is terminated when the SOx poisoningof the sub NOx catalyst 61 has been eliminated.

This is because it is expected that when the SOx poisoning regenerationcontrol of the main NOx catalyst 55 and the SOx poisoning regenerationcontrol of the sub NOx catalyst 61 are simultaneously terminated, theNOx catalyst of which the SOx poisoning has been eliminated first isexposed to high temperature until the SOx poisoning of the other NOxcatalyst is eliminated, thereby causing thermal deterioration of thecatalyst.

Further, the sixth embodiment differs from the fifth embodiment in that,while in the fifth embodiment the control is returned to normal as soonas the SOx poisoning regeneration of the main and sub NOx catalysts 55and 61 is completed, in this sixth embodiment, when the SOx poisoningregeneration of the main and sub NOx catalysts 55 and 61 is completed,the control is returned to normal after temporarily cooling the main andsub NOx catalysts 55 and 61.

When eliminating the SOx poisoning of the main and sub NOx catalysts 55and 61, the temperature of the SOx poisoning of the main and sub NOxcatalysts 55 and 61 is raised to a very high temperature range, so thatwhen the control is returned to normal immediately after the eliminationof the SOx poisoning and the flow of exhaust to the main NOx catalyst 55and the sub NOx catalyst 61 is cut off, the the main NOx catalyst 55 andthe sub NOx catalyst 61 are left at the high temperature, therebycausing thermal deterioration in the main NOx catalyst 55 and the subNOx catalyst 61.

In the following, the SOx poisoning regeneration control of the sixthembodiment will be described with reference to the flowchart of FIG. 14.The flowchart of FIG. 14 shows a SOx poisoning regeneration controlroutine. This SOx poisoning regeneration control routine is stored inadvance in the ROM 32 of the ECU 30, and is repeatedly executed by theCPU 34 for each predetermined time.

(Step 701)

In the SOx poisoning regeneration control routine, the ECU 30 firstexecutes in Step 701 the SOx poisoning degree determination processingon the main NOx catalyst 55.

(Step 702)

In Step 702, the ECU 30 determines whether the degree of SOx poisoningof the main NOx catalyst 55 determined in step 701 exceeds apredetermined reference value or not.

When determined negative in Step 702, the ECU 30 determines that thereis no need to execute SOx poisoning regeneration processing on the mainNOx catalyst 55, and the process advances to Step 703.

(Step 703)

In Step 703, the ECU 30 executes the SOx poisoning degree determiningprocessing on the sub NOx catalyst 61.

(Step 704)

In Step 704, the ECU 30 determines whether the degree of SOx poisoningof the sub NOx catalyst determined in Step 703 exceeds a predeterminedreference value or not.

When determined negative in Step 704, the ECU 30 determines that thereis no need to execute SOx poisoning regeneration processing on the subNOx catalyst 61, and temporarily terminates the execution of theroutine.

On the other hand, when determined affirmative in Step 702 or Step 704,that is, when it is determined that it is necessary to execute the SOxpoisoning regeneration processing on the main NOx catalyst 55 or the subNOx catalyst 61, the process by the ECU 30 advances to Step 705.

(Step 705)

In Step 705, the ECU 30 simultaneously starts the execution of SOxpoisoning regeneration processing on the main NOx catalyst 55 and theexecution of the SOx poisoning regeneration processing on the sub NOxcatalyst 61. More specifically, the ECU 30 controls the first actuator62 so as to retain the first exhaust switching valve 63 in the fullyopen state, controls the second actuator 64 so as to retain the secondexhaust switching valve 65 in the fully open state, and further executesNOx catalyst temperature raising processing so as to raise thetemperature of the main and sub NOx catalysts 55 and 61 to apredetermined temperature range (500° C. to 700° C.).

Examples of the method of NOx catalyst temperature raising processinginclude the following methods: (1) The internal combustion engine 1 isoperated at rich air-fuel ratio so as to make the air-fuel ratio of theexhaust rich, and secondary air is supplied to the portion of theexhaust in the exhaust passage on the upstream side of the main and subNOx catalysts 55 and 61, whereby a sufficient amount of unburned fuelcomponents and oxygen is supplied to the main and sub NOx catalysts 55and 61. The unburned fuel components and oxygen are caused to oxidize(burn) in the main and sub NOx catalysts 55 and 61, thereby rapidlyraising the temperature of the main and sub NOx catalysts 55 and 61. (2)The internal combustion engine 1 is operated at stoichiometric or richair-fuel ratio and the main and sub NOx catalysts 55 and 61 are heatedby a dedicated heater. (3) Part of the cylinders of the internalcombustion engine 1 is operated at rich air-fuel ratio, and the rest ofthe cylinders is operated at lean air-fuel ratio, whereby a gas air-fuelmixture of an exhaust containing a sufficient amount of unburned fuelcomponents and an exhaust containing a sufficient amount of oxygen issupplied to the main and sub NOx catalysts 55 and 61; the unburned fuelcomponents and oxygen contained in the gas air-fuel mixture are causedto undergo oxidization in the main and sub NOx catalysts 55 and 61,thereby raising the temperature of the main and sub NOx catalysts 55 and61.

When the above-described SOx poisoning regeneration processing isexecuted, the exhaust from the internal combustion engine 1 flowsthrough both the main NOx catalyst 55 and the sub NOx catalyst 61, andthe spatial velocity of the exhaust flowing through the main and sub NOxcatalysts 61 is reduced as compared with the case in which the exhaustis caused to flow through only one of the main NOx catalyst 55 and thesub NOx catalyst 61.

As a result, the SOx poisoning regeneration efficiency of the main andsub NOx catalysts 55 and 61 improves, so that it is possible to reducethe fuel consumption amount related to the SOx poisoning regenerationprocessing, and to reduce the length of time during which the main andsub NOx catalysts 55 and 61 are exposed to high temperature.

(Step 706)

In Step 706, the ECU 30 determines whether the SOx poisoningregeneration of the main NOx catalyst 55 has been completed or not.Examples of the method for determining the SOx poisoning regenerationcompletion of the main NOx catalyst 55 include the following methods:(1) a method in which the relationship between the degree of SOxpoisoning of the main NOx catalyst 55 and the required time for the SOxpoisoning regeneration (main NOx catalyst SOx poisoning regenerationtime) is obtained in advance by experiment, and it is determined thatthe SOx poisoning of the main NOx catalyst 55 has been eliminated whenthe execution time of the SOx poisoning regeneration processing exceedsthe SOx poisoning regeneration time for the main NOx catalyst; and (2) amethod in which SOx sensors adapted to output an electric signalcorresponding to the SOx concentration in the exhaust is arranged in theexhaust pipe 57 on the downstream side of the main NOx catalyst 55, andit is determined that the SOx poisoning of the main NOx catalyst 55 hasbeen eliminated when the output signal value of the SOx sensor hasbecome smaller than a predetermined value.

(Step 707)

When determined affirmative in Step 706, the process by the ECU 30advances to Step 707, in which whether the SOx poisoning regeneration ofthe sub NOx catalyst 61 has been completed or not is determined.

Examples of the method for determining the SOx poisoning regenerationcompletion of the sub NOx catalyst 61 include the following methods: (1)a method in which the relationship between the degree of SOx poisoningof the sub NOx catalyst 61 and the required time for the SOx poisoningregeneration (sub NOx catalyst SOx poisoning regeneration time) isobtained in advance by experiment, and it is determined that the SOxpoisoning of the sub NOx catalyst 61 has been eliminated when theexecution time of the SOx poisoning regeneration processing exceeds theSOx poisoning regeneration time for the sub NOx catalyst; and (2) amethod in which and the SOx sensor adapted to output an electric signalcorresponding to the SOx concentration in the exhaust is arranged in thebypass path 59 on the downstream side of the sub NOx catalyst 61, and itis determined that the SOx poisoning of the sub NOx catalyst 61 has beeneliminated when the deatection output signal value of the SOx sensor hasbecome smaller than a predetermined value.

(Step 708)

When determined negative in Step 707, that is, when the SOx poisoningregeneration of the main NOx catalyst 55 has been completed and the SOxpoisoning regeneration of the sub NOx catalyst 61 has not been completedyet, the process by the ECU 30 advances to Step 708, in which theexecution of the SOx poisoning regeneration processing on the sub NOxcatalyst 61 is temporarily terminated, and the cooling of the main NOxcatalyst 55 is executed for a predetermined period of time.

More specifically, the ECU 30 interrupts the execution of the NOxcatalyst temperature rise processing, and controls the second actuator64 so as to change only the second exhaust switching valve 65 of thefirst and second exhaust switching valves 63 and 65, which are in thefully open state, from the fully open state to the totally closed state.

In this case, the exhaust discharged from the internal combustion engine1 flows through the main NOx catalyst 55, and does not flow through thesub NOx catalyst 61. When the exhaust passes through the main NOxcatalyst 55, the heat of the main NOx catalyst 55 is taken by theexhaust, resulting in a reduction in the temperature of the main NOxcatalyst 55.

In the process, it might be possible to operate the internal combustionengine 1 at the lean air-fuel ratio to lower the temperature of theexhaust. However, immediately after the execution of the SOx poisoningregeneration processing, the temperature of the main NOx catalyst 55 canbe higher than the catalyst purifying wind. If in such a condition theinternal combustion engine 1 is operated at lean air-fuel ratio, the NOxin the exhaust is not purified by the main NOx catalyst 55, resulting ina deterioration in exhaust emission.

In view of this, in the sixth embodiment, when cooling the main NOxcatalyst 55 after the execution of the SOx poisoning regenerationprocessing, the internal combustion engine 1 is operated at thestoichiometric air-fuel ratio, and the main NOx 55 is cooled whilerestraining the deterioration in exhaust emission.

(Step 709)

When the ECU 30 executed the above main NOx catalyst cooling processingfor a predetermined period of time, the process advances to Step 709. InStep 709, the ECU 30 terminates the main NOx catalyst coolingprocessing, and resumes the SOx poisoning regeneration processing of thesub NOx catalyst 61.

More specifically, the ECU 30 controls the first actuator 62 so as toswitch the first exhaust switching valve 63 from the fully open state tothe totally closed state, and controls the second actuator 64 so as toswitch the second exhaust switching valve 65 from the totally closedstate to the fully open state, and further resumes the execution of theNOx catalyst temperature rise processing so as to raise the temperatureof the sub NOx catalyst 65.

(Step 710)

In Step 710, the ECU 30 determines whether the SOx poisoningregeneration of the sub NOx catalyst 61 has been completed or not.

When determined negative in Step 710, the process by the ECU 30 isreturned to Step 709 in order to continuously execute the SOx poisoningregeneration processing on the sub NOx catalyst 61. On the other hand,when determined affirmative in Step 710, the process by the ECU 30advances to Step 711.

(Step 711)

In Step 711, the ECU 30 terminates the execution of the SOx poisoningregeneration processing on the sub NOx catalyst 61, and executes thecooling of the sub NOx catalyst 61 for a predetermined period of time.More specifically, the ECU 30 terminates the execution of the NOxcatalyst temperature raising processing, and switches the operatingcondition of the internal combustion engine 1 to the stoichiometricoperation while retaining the first exhaust switching valve 63 in thetotally closed state, and the second exhaust switching valve 65 in thefully open state.

In this case, the exhaust discharged from the internal combustion engine1 flows through the sub NOx catalyst 61, and does not flow through themain NOx catalyst 55. When the exhaust passes through the sub NOxcatalyst 61, the heat of the sub NOx catalyst 61 is taken by theexhaust, and the temperature of the sub NOx catalyst 61 is lowered.

(Step 712)

When the ECU 30 has executed the above-described sub NOx catalystcooling processing for a predetermined period of time, the processadvances to Step 712. In Step 712, the ECU 30 returns the control of theinternal combustion engine 1 and the control of the main and sub NOxcatalysts 55 and 61 to normal. After the execution of the processing ofStep 712, the ECU 30 temporarily terminates the execution of theroutine.

On the other hand, when determined affirmative in Step 707, that is,when the SOx poisoning regeneration of both the main NOx catalyst 55 andthe sub NOx catalyst 61 is completed substantially simultaneously, theprocess by the ECU 30 advances to Step 713.

(Step 713)

In Step 713, the ECU 30 terminates the execution of the SOx poisoningregeneration processing of the main and sub NOx catalysts 55 and 61, andexecutes the cooling of the main NOx catalyst and the sub NOx catalystfor a predetermined period of time in order to simultaneously cool themain and sub NOx catalysts 55 and 61. More specifically, the ECU 30terminates the execution of the NOx catalyst temperature raisingprocessing, and switches the operating condition of the internalcombustion engine 1 to the stoichiometric operation while retaining thefirst exhaust switching valve 63 in the fully open state, and the secondexhaust switching valve 65 in the fully open state.

In this case, the exhaust discharged from the internal combustion engine1 flows through both the main NOx catalyst 55 and the sub NOx catalyst61. When the exhaust passes through the main and sub NOx catalysts 55and 61, the heat of the main and sub NOx catalysts 55 and 61 is taken bythe exhaust, and the temperature of the main and sub NOx catalysts 55and 61 is lowered.

When the ECU 30 has executed the cooling of the main NOx catalyst andthe sub NOx catalyst for a predetermined period of time, the processadvances to Step 712, in which the control of the internal combustionengine 1 and the control of the first and second exhaust switchingvalves 63 and 65 are returned to normal, and the execution of theroutine is temporarily terminated.

(Step 714)

Next, when determined negative in Step 706, the process by the ECU 30advances to Step 714, in which whether the SOx poisoning regeneration ofthe main NOx catalyst 55 has been completed or not is determined.

When determined negative Step 714, that is, when the SOx poisoningregeneration of both the main and sub NOx catalysts 55 and 61 has notbeen completed yet, the process by the ECU 30 returns to Step 705, andthe SOx poisoning regeneration processing on the main and sub NOxcatalysts 55 and 61 is continued. On the other hand, when determinedaffirmative in Step 714, that is, when the SOx poisoning regeneration ofthe main NOx catalyst 55 has not been completed yet, and the SOxpoisoning regeneration of the sub NOx catalyst 61 has been completed,the process by the ECU 30 advances to Step 715.

(Step 715)

In Step 715, the ECU 30 temporarily interrupts the execution of the SOxpoisoning regeneration processing on the main NOx catalyst 55, andexecutes the cooling of the sub NOx catalyst 61 for a predeterminedperiod of time. More specifically, the ECU 30 interrupts the executionof the NOx catalyst temperature raising processing, and controls thefirst actuator 62 in order to switch only the first exhaust switchingvalve 63 of the first and second exhaust switching valves 63 and 65,which are in the fully open state, to the totally closed state, andswitches the operating condition of the internal combustion engine 1 tothe stoichiometric operation.

In this case, the exhaust discharged from the internal combustion engine1 flows through the sub NOx catalyst 61, and does not flow through themain NOx catalyst 55. When the exhaust flows through the sub NOxcatalyst 61, the heat of the sub NOx catalyst 61is taken by the exhaust,and the temperature of the sub NOx catalyst 61 is lowered.

(Step 716)

When the ECU 30 has executed the cooling of the sub NOx catalyst for apredetermined period of time, the procedure advances to Step 716. InStep 716, the ECU 30 terminates the execution of the cooling of the subNOx catalyst, and resumes the execution of the SOx poisoningregeneration processing on the main NOx catalyst 55. More specifically,the ECU 30 controls the second actuator 64 so as to switch the secondexhaust switching valve 65 from the fully open state to the totallyclosed state, and controls the first actuator 62 so as to switch thefirst exhaust switching valve 63 from the totally closed state to thefully open state, and further re-starts the execution of the NOxcatalyst temperature raising processing so as to raise the temperatureof the main NOx catalyst 55.

(Step 717)

In Step 717, the ECU 30 determines whether the SOx poisoningregeneration of the main NOx catalyst 55 has been completed or not.

When determined negative in Step 717, the process by the ECU 30 returnsto Step 716 in order to continuously execute the SOx poisoningregeneration processing on the main NOx catalyst 55. On the other hand,when determined affirmative in Step 717, the process by the ECU 30advances to Step 718.

(Step 718)

In Step 718, the ECU 30 completes the execution of the SOx poisoningregeneration processing on the main NOx catalyst 55, and executes thecooling of the main NOx catalyst 55 for a predetermined period of time.More specifically, the ECU 30 terminates the NOx catalyst temperatureraising processing, and switches the operating condition of the internalcombustion engine 1 to the stoichiometric operation while retaining thefirst exhaust switching valve 63 in the fully open state, and the secondexhaust switching valve 65 in the totally closed state.

In this case, the exhaust discharged from the internal combustion engine1 flows through the main NOx catalyst 55, and does not flow through thesub NOx catalyst 61. When the exhaust flows through the main NOxcatalyst 55, the heat of the main NOx catalyst 55 is taken by theexhaust, and the temperature of the main NOx catalyst is lessed.

When the ECU 30 executes the main NOx catalyst cooling processing for apredetermined period of time, the procedure advances to Step 712, andthe control of the internal combustion engine 1 and the control of themain and sub NOx catalysts 55 and 61 are returned to normal, and theexecution of the routine is temporarily terminated.

In the sixth embodiment described above, when the SOx poisoning degreeof at least one of the main NOx catalyst 55 and the sub NOx catalyst 61exceeds a predetermined reference value, the SOx poisoning regenerationcontrol of the main NOx catalyst 55 and the SOx poisoning regenerationcontrol of the sub NOx catalyst 61 are simultaneously conducted, sothat, as compared with the case in which the main NOx catalyst 55 andthe sub NOx catalyst 61 are individually regenerated, it is possible toreduce the execution frequency of the SOx poisoning regenerationcontrol.

Further, in the sixth embodiment, in the SOx poisoning regenerationcontrol of the main and sub NOx catalysts 55 and 61, the exhaust fromthe internal combustion engine 1 is caused to flow through both the mainand sub NOx catalysts 55 and 61, so that, as compared with the case inwhich the exhaust is caused to flow only through one of the main NOxcatalyst 55 and the sub NOx catalyst 61, the spatial velocity of theexhaust in the NOx catalysts 55 and 61 is reduced, and the SOxregeneration efficiency is improved, making it possible to reduce theexecution time of the SOx poisoning regeneration control.

Further, in the sixth embodiment, the SOx poisoning regeneration controlof the main NOx catalyst 55 is terminated when the SOx poisoning of themain NOx catalyst is eliminated, and the SOx poisoning regenerationcontrol of the sub NOx catalyst 61 is terminated when the SOx poisoningof the sub Nox catalyst 61 is eliminated, so that there is no danger ofthe NOx catalyst of which SOx poisoning is eliminated earlier beingunnecessarily exposed to high temperature.

Further, in the sixth embodiment, when the SOx poisoning regeneration ofthe main NOx catalyst 55 and the sub NOx catalyst 61 is completed, themain NOx catalyst 55 and the sub NOx catalyst 61 are cooled, so that themain NOx catalyst 55 and the sub NOx catalyst 61 are not left at hightemperature, thereby preventing thermal deterioration in the main andsub NOx catalysts 55 and 61.

Thus, in this sixth embodiment, in addition to the effect of the fifthembodiment described above, it is advantageously possible to preventthermal deterioration of the main and sub NOx catalysts 55 and 61 due tothe SOx poisoning regeneration control.

Seventh Embodiment

Next, a seventh embodiment of the exhaust emission purifying device foran internal combustion engine of the present invention will bedescribed. Here, the construction differing from that of the thirdembodiment will be described, and a description of the constructionsimilar to that of the third embodiment will be omitted.

The seventh embodiment differs from the third embodiment in that, whilein the third embodiment when the temperature of the main NOx catalyst 55becomes higher than a predetermined temperature when the engineoperating condition is in the lean/rich spike control execution range,the NOx catalyst temperature rise restraining control is immediatelyexecuted in order to prevent excessive temperature rise in the main NOxcatalyst 55, in this seventh embodiment, when the temperature of themain NOx catalyst 55 becomes higher than a predetermined temperaturewhen the engine operating condition is in the lean/rich spike controlexecution range, the NOx catalyst temperature rise restraining controlis executed after the SOx poisoning regeneration control of the sub NOxcatalyst 61 is executed.

In normal control of the exhaust emission purifying device, when theengine operating condition is in the stoichiometric control executionrange or the rich air-fuel ratio control execution range, and thetemperature of the 3-way catalyst 51 satisfies the SOx releasingcondition, the first exhaust switching valve 63 is retained in thetotally closed state, and the second exhaust switching valve 65 isretained in the fully open state so as to prevent the SOx released fromthe 3-way catalyst 51 from flowing into the main NOx catalyst 55, andthe entire amount of exhaust flows through the sub NOx catalyst 61, sothat the sub NOx catalyst 61 is more subject to the SOx poisoning thanthe main NOx catalyst 55, and when the NOx catalyst temperature riserestraining control is executed with the sub NOx catalyst 61 undergoingthe SOx poisoning, the NOx contained in the exhaust flowing through thesub NOx catalyst 61 is not sufficiently purified, resulting in adeterioration in exhaust emission.

In the following, the catalyst temperature rise restraining control ofthe seventh embodiment will be described with reference to the flowchartof FIG. 15. The flowchart of FIG. 15 shows a NOx catalyst temperaturerise restraining control routine. This NOx catalyst temperature riserestraining control routine is stored in the ROM 32 of the ECU 30 inadvance, and is repeatedly executed by the CPU 34 for each predeterminedtime.

(Step 801)

In the catalyst temperature rise restraining control routine, the ECU 30first determines in Step 801 whether the operating condition of theinternal combustion engine 1 is in the lean/rich spike control executionrange or not.

(Step 802)

When determined affirmative in Step 801, the process by the ECU 30advances to Step 802, in which a determination is made as to whether theoutput signal value (exhaust temperature) of the temperature sensor 66is higher than a pre-set upper limit value T₁ (for example, the upperlimit value of the catalyst purification wind of the main NOx catalyst55) or not.

(Step 803)

When determined affirmative in Step 802, the ECU 30 determines that itis necessary to execute NOx catalyst temperature rise restrainingprocessing so as to prevent excessive temperature rise in the main NOxcatalyst 55, and the procedure advances to Step 803. In Step 803, theECU 30 executes the SOx poisoning degree determining processing on thesub NOx catalyst 61. Examples of the method of determining the SOxpoisoning degree of the sub NOx catalyst 61 include one in whichestimation is made from the operation history of the internal combustionengine 1, etc.

(Step 804)

In Step 804, the ECU 30 determines whether the SOx poisoning degree ofthe sub NOx catalyst 61 is less than a predetermined reference value ornot. The reference value is a value obtained in advance by experiment,and is stored in the ROM 32.

(Step 805)

When determined affirmative in Step 804, the ECU 30 determines thatthere is no need to perform the SOx poisoning regeneration of the subNOx catalyst 61, and the procedure advances to Step 805. In Step 805,the ECU 30 executes NOx catalyst temperature rise restraining processingon the main NOx catalyst 55.

More specifically, the ECU 30 controls the first actuator 62 and thesecond actuator 64 so as to retain the first and second exhaustswitching valves 63 and 65 in the fully open state, causing the exhaustto flow through the main NOx catalyst 55 and the sub NOx catalyst 61.

In this case, the exhaust from the internal combustion engine 1 flowsthrough both the main NOx catalyst 55 and the sub NOx catalyst 61, andthe amount of exhaust flowing through the main NOx catalyst 55 issmaller than that in the case in which the exhaust from the internalcombustion engine 1 flows solely through the main NOx catalyst 55, sothat the quantity of heat the main NOx catalyst 55 receives from theexhaust decreases, and there is no excessive temperature rise in themain NOx catalyst 55.

Further, when the amount of exhaust flowing through the main NOxcatalyst 55 decreases as a result of the exhaust from the internalcombustion engine 1 flowing through both the main NOx catalyst 55 andthe sub NOx catalyst 61, the spatial velocity of the exhaust in the mainNOx catalyst 55 is reduced, so that the NOx purification ratio of themain NOx catalyst 55 improves. Similarly, the amount of exhaust flowingthrough the sub NOx catalyst 61 is smaller than that in the case inwhich the exhaust from the internal combustion engine 1 flows solelythrough the sub NOx catalyst 61, so that the spatial velocity of theexhaust in the sub NOx catalyst 61 is also reduced, and the NOxpurification ratio of the sub NOx catalyst 61 is also improved.

(Step 806)

In Step 806, the ECU 30 determines whether the output signal value(exhaust temperature) of the temperature sensor 66 has become lower thana predetermined temperature T₂ or not. The predetermined temperature T₂is a value smaller than the upper limit value T₁ and not smaller thanthe lower limit value of the catalyst purification wind of the main NOxcatalyst 55.

When determined negative in Step 806, the process by the ECU 30 returnsto Step 805 to continue the execution of the NOx catalyst temperaturerise restraining processing. On the other hand, when determinedaffirmative in Step 806, the process by the ECU 30 advances to Step 807.

(Step 807)

In Step 807, the ECU 30 terminates the execution of the NOx catalysttemperature rise restraining processing, and returns the control of thefirst and second exhaust switching valves 63 and 65 to normal. When theprocessing of Step 807 has been executed, the ECU 30 temporarilyterminates the execution of the routine.

On the other hand, when determined negative in Step 804, the ECU 30determines that it is necessary to perform the SOx poisoningregeneration on the sub NOx catalyst 61 before executing the NOxcatalyst temperature rise restraining processing, and the procedureadvances to Step 808.

(Step 808)

In Step 808, the ECU 30 executes the SOx poisoning regenerationprocessing on the sub NOx catalyst 61. More specifically, the ECU 30controls the first actuator 62 so as to bring the first exhaustswitching valve 63 into the totally closed state, and controls thesecond actuator 64 so as to bring the second exhaust switching valve 65into the fully open state, and further executes the NOx catalysttemperature raising processing so as to raise the temperature of the subNOx catalyst 61 to a predetermined temperature range (500° C. to 700°C.).

Examples of the method of NOx catalyst temperature raising processinginclude the following methods: (1) The internal combustion engine 1 isoperated at rich air-fuel ratio so as to make the air-fuel ratio of theexhaust rich, and the secondary air is supplied to the portion of theexhaust in the exhaust passage on the upstream side of the main and subNOx catalysts 55 and 61, whereby a sufficient amount of unburned fuelcomponents and oxygen is supplied to the main and sub NOx catalysts 55and 61. The unburned fuel components and oxygen are caused to oxidize(burn) in the main and sub NOx catalysts 55 and 61, thereby rapidlyraising the temperature of the main and sub NOx catalysts 55 and 61. (2)The internal combustion engine 1 is operated at stoichiometric or richair-fuel ratio and the main and sub NOx catalysts 55 and 61 are heatedby a dedicated heater. (3) Part of the cylinders of the internalcombustion engine 1 is operated at rich air-fuel ratio, and the rest ofthe cylinders is operated at lean air-fuel ratio, whereby a gas air-fuelmixture of an exhaust containing a sufficient amount of unburned fuelcomponents and an exhaust containing a sufficient amount of oxygen issupplied to the main and sub NOx catalysts 55 and 61; the unburned fuelcomponents and oxygen contained in the gas air-fuel mixture are causedto undergo oxidization in the main and sub NOx catalysts 55 and 61,thereby raising the temperature of the main and sub NOx catalysts 55 and61.

(Step 809)

In Step 809, the ECU 30 determines whether the SOx poisoningregeneration of the sub NOx catalyst 61 has been completed or not.Examples of the method for determining the SOx poisoning regenerationcompletion of the sub NOx catalyst 61 include the following methods: (1)a method in which the relationship between the degree of SOx poisoningof the sub NOx catalyst 61 and the required time for the SOx poisoningregeneration (sub NOx catalyst SOx poisoning regeneration time) isobtained in advance by experiment, and it is determined that the SOxpoisoning of the sub NOx catalyst 61 has been eliminated when theexecution time of the SOx poisoning regeneration processing exceeds theSOx poisoning regeneration time for the sub NOx catalyst 61; and (2) amethod in which a SOx sensor adapted to output an electric signalcorresponding to the SOx concentration in the exhaust is arranged in thebypass pipe 59 on the downstream side of the sub NOx catalyst 61, and itis determined that the SOx poisoning of the sub NOx catalyst 61 has beeneliminated when the detected output signal value of the SOx sensor hasbecome smaller than a predetermined value.

When determined negative in Step 809, the process by the ECU 30 returnsto Step 808, in which the execution of the SOx poisoning regenerationprocessing of the sub NOx catalyst 61 is continued. On the other hand,when determined affirmative in Step 809, the ECU 30 successivelyexecutes the processings of Step 805, Step 806, and Step 807 to preventexecessive temperature rise in the main NOx catalyst 55.

In the seventh embodiment described above, in addition to the effect ofthe above-described third embodiment, it is possible to improve the NOxpurification ratio of the main NOx catalyst 55 and the sub NOx catalyst61 while preventing excessive temperature rise in the main NOx catalyst55.

Eighth Embodiment

Next, an eighth embodiment of the exhaust emission purifying device foran internal combustion engine of the present invention will be describedwith reference to FIG. 16. Here, the construction differing from that ofthe third embodiment will be described, and a description of theconstruction similar to the third embodiment will be omitted.

The eighth embodiment differs from the third embodiment in that, whilein the third embodiment the exhaust leaking to the sub NOx catalyst 61through the second exhaust switching valve 65 is not taken into accountwhen estimating the amount of NOx absorbed by the main NOx catalyst 55in the lean/rich spike control, in this eighth embodiment, the amount ofNOx absorbed by the main NOx catalyst 55 is estimated while taking intoaccount the exhaust leaked to the sub NOx catalyst 61.

When the amount of NOx absorbed by the main NOx catalyst 55 is estimatedwithout taking into account the exhaust leaking to the sub NOx catalyst61, it is to be expected that the estimated value will be larger thanthe actual NOx absorption amount. If the lean/rich spike control isexecuted on the basis of such an estimated value, rich spike control isexecuted despite the fact that the NOx absorbing capacity of the mainNOx catalyst 55 is not saturated yet, so that it is impossible toefficiently utilize the NOx absorbing capacity of the main NOx catalyst55, unnecessarily increasing the execution frequency of the rich spikecontrol to thereby cause a deterioration in the fuel consumption amount.

Further, the eighth embodiment differs from the third embodiment inthat, while in the third embodiment the lean/rich spike control isexecuted solely for the main NOx catalyst 55, in this eighth embodiment,the lean/rich spike control is executed for both the main NOx catalyst55 and the sub NOx catalyst 61.

This is because of the fact when the engine operating condition is inthe lean/rich spike control execution range, part of the exhaust leaksto the sub NOx catalyst 61 through the second exhaust switching valve 65to cause NOx in the exhaust to be absorbed by the sub NOx catalyst 61,so that it is necessary to release and clean the NOx absorbed by the subNOx catalyst 61.

In the following, the lean/rich spike control in the eighth embodimentwill be specifically described. The flowchart of FIG. 16 shows alean/rich spike control routine. This lean/rich spike control routine isstored in advance in the ROM 32 of the ECU 30, and is repeatedlyexecuted for each predeterrmined time by the CPU 34.

(Step 901)

In the lean/rich spike control routine, the ECU 30 first gains access inStep 901 to a first NOx release flag storage region set previously in apredetermined region of the RAM 33, and determines whether “1” is storedor not.

The first NOx release flag storage region is a region in which “1” isstored when the amount of NOx absorbed by the main NOx catalyst 55 ishigher than the NOx limit value that can be absorbed by the main NOxcatalyst 55, and in which “0” is stored when the amount of NOx absorbedby the main NOx catalyst 55 is lower than the limit value.

(Step 902)

When determined affirmative in Step 901, that is, when it is determinedthat “0” is stored in the first NOx release flag storage region of theRAM 33, the ECU 30 proceeds to Step 902. In Step 902, the ECU 30 gainsaccess to a second NOx release flag storage region previously set in apredetermined region of the RAM 33, and determines whether “1” is storedor not.

A second NOx release flag storage region is a region in which “1” isstored when the amount of NOx absorbed by the sub NOx catalyst 61 ishigher than the NOx limit value that can be absorbed by the sub NOxcatalyst 61, and in which “0” is stored when the amount of NOx absorbedby the sub NOx catalyst 61 is less than the limit value.

(Step 903)

When determined affirmative in Step 902, that is, when it is determinedthat “0” is stored in the second NOx release flag storage region of theRAM 33, the process by the ECU 30 advances to Step 903. In Step 903, theECU 30 determines whether the engine operating condition is in the leanair-fuel ratio control execution range or not.

(Step 904)

When determined affirmative in Step 903, the ECU 30 proceeds to Step904. In Step 904, the entire amount of NOx absorbed by the main NOxcatalyst 55 and the entire amount of NOx absorbed by the sub NOxcatalyst 61 are computed on the basis of the amount of exhaust leakingto the sub NOx catalyst 61 through a second exhaust switching valve 65.

More specifically, the ECU 30 first calculates the amount of NOxdischarged from the internal combustion engine 1 during a fixed period(hereinafter referred to as engine discharge NOx amount) using theengine speed, fuel injection amount, etc. as parameters. An example of amethod for calculating the engine discharge NOx amount is thatcalculation is made using the engine speed, intake air amount, and fuelinjection amount as parameters. It is also possible to obtain arelationship between the engine speed, intake air amount, fuel injectionamount, and the engine discharge NOx amount in advance by experiment,and store the relationship in the ROM 32 in the form of a map.

Subsequently, the ECU 30 calculates the amount of NOx leaked to the subNOx catalyst 61 in a fixed period of time (hereinafter referred to asNOx leak amount) . An example of a method for calculating the NOx leakamount is that calculation is made using an exhaust flow rate and theengine exhaust NOx amount as parameters since the NOx leak amount isconsidered to vary according to the exhaust flow rate (exhaust pressure)and the engine exhaust NOx amount.

It is also possible to obtain the relationship between the exhaust flowrate, the engine exhaust NOx amount, and the NOx leak amount in advanceby experiment, storing the relationship in the ROM 32 in the form of amap. Further, since the exhaust pressure and the exhaust flow rate canbe estimated from the parameters indicating the engine operatingcondition, such as engine speed and intake air amount, theabove-mentioned map may be one showing the relationship between theengine operating condition, the engine exhaust NOx amount, and the NOxleak amount.

The ECU 30 calculates the engine exhaust NOx amount and the NOx leakamount by the above-described method. The ECU 30 subtracts the NOx leakamount from the engine exhaust NOx amount to thereby calculate the NOxabsorption amount of the main NOx catalyst 55. The ECU 30 adds the thuscalculated NOx absorption amount to a counter value of a firstabsorption counter C1.

The first absorption counter C1 is formed by a storage region set in apredetermined region of the RAM 33, or a register or the like containedin the CPU 34, and retains the integrated value of the amount of NOxabsorbed by the main NOx catalyst 55, in other words, the entire amountof NOx absorbed by the main NOx catalyst 55.

On the other hand, the ECU 30 adds the NOx leak amount to the countervalue of a second absorption counter C2. The second absorption counterC2 is formed of a storage region set in a predetermined region of theRAM 33, a register contained in the CPU 34 or the like, and retains theintegrated value of the amount of NOx absorbed by the sub NOx catalyst61, that is, the entire amount of NOx absorbed by the sub NOx catalyst61.

(Step 905)

The ECU 30 reads the counter value :C1 of the first absorption counterC1 updated in Step 904, and compares the counter value :C1 with thelimit value :C1MAX of the amount of NOx that can be absorbed by the mainNOx catalyst 55. More specifically, the ECU 30 determines whether thecounter value :C1 is less than the limit value :C1MAX or not.

(Step 906)

When determined affirmative in Step 905, the ECU 30 determines that theentire NOx absorption amount of the main NOx catalyst 55 has not reachedthe limit value and that there is no need to execute rich spike controlon the main NOx catalyst 55, and the procedure advances to Step 906. InStep 906, the ECU 30 reads the counter value :C2 of the secondabsorption counter C2 updated in Step 904, and compares the countervalue :C2 with the limit value :C2MAX of the amount of NOx that can beabsorbed by the sub NOx catalyst 61. More specifically, the ECU 30 makesa determination as to whether the counter value :C2 is less than thelimit value :C2MAX.

When determined affirmative in Step 906, the ECU 30 determines that theentire NOx absorption amount of the sub NOx catalyst 61 has not reachedthe limit value yet, and that there is no need to execute the rich spikecontrol on the sub NOx catalyst 61, and temporarily terminates theexecution of the routine.

(Step 907)

Next, when determined negaative in Step 903, the ECU 30 determines thatthe engine operating condition is not in the lean air-fuel ratio controlexecution range, in other words, the engine operating condition is inthe stoichiometric control execution range (or rich air-fuel ratiocontrol execution range), and that the first exhaust switching valve 63is retained in the totally closed state and the second exhaust switchingvalve 65 in the fully open state, and the procedure advances to Step907. In Step 907, the ECU 30 computes the entire amount of NOx releasedfrom the main NOx catalyst 55 and the entire amount of NOx released fromthe sub NOx catalyst 61 on the basis of the amount of exhaust leaked tothe main NOx catalyst 55 through the first exhaust switching valve 63.

That is, when the engine operating condition is in the stoichiometriccontrol execution range (or the rich air-fuel ratio control executionrange), the first exhaust switching valve 63 is retained in the totallyclosed state and the second exhaust switching valve 65 is retained inthe fully open state, so that the exhaust of stoichiometric or richair-fuel ratio discharged from the internal combustion engine 1 mainlyflows through the sub NOx catalyst 61. However, since the sealingproperty of the first exhaust switching valve 63 is not perfect, a smallamount of exhaust leaks to the main NOx catalyst 55 through the firstexhaust switching valve 63.

Thus, it is assumed that when the engine operating condition is in thestoichiometric control or rich air-fuel ratio control execution range,most of the exhaust flows through the sub NOx catalyst 61, so that theNOx that has been absorbed by the sub NOx catalyst 61 is released andreduced, and the remaining small amount of exhaust flows through themain NOx catalyst 55, and the NOx that has been absorbed by the main NOxcatalyst 55 is released and reduced.

Thus, the ECU 30 first calculates the amount of unburned fuel componentsdischarged from the internal combustion engine 1 during a fixed periodof time (herein after referred to as the engine exhaust fuel componentamount) by using the engine speed, intake air amount, etc. asparameters. An example of a method for calculating the engine exhaustfuel component amount is that calculation is performed by using theengine speed, intake air amount, and fuel injection amount asparameters. It is also possible to obtain a relationship between theengine speed, intake air amount, and fuel injection amount in advance byexperiment, and to store the relationship in the ROM 32 in the form of amap.

Subsequently, the ECU 30 calculates the amount of unburned fuelcomponents leaking to the main NOx catalyst 55 during a fixed period oftime, that is, the amount of unburned fuel components flowing into themain NOx catalyst 55 during a fixed period of time (hereinafter referredto as the main fuel component amount). The ECU 30 subtracts the mainfuel component amount from the engine exhaust fuel component amount tothereby calculate the amount of unburned fuel components (sub fuelcomponent amount) flowing into the sub NOx catalyst 61.

The ECU 30 calculates the amount of NOx released and reduced when themain fuel component amount flows into the main NOx catalyst 55(hereinafter referred to as the first NOx release amount), and theamount of NOx released and reduced when the sub fuel component amountflows into the sub NOx catalyst 61 (hereinafter referred to as thesecond NOx release amount).

The ECU 30 adds the first NOx release amount calculated as describedabove to the counter value of a first release counter CC1, and adds thesecond NOx release amount to a second release counter CC2.

The first release counter CC1 is formed by a storage region set in apredetermined region of the PAM 33, a register contained in the CPU 34or the like, and retains the integrated value of the amount of NOxreleased and reduced in the main NOx catalyst 55, in other words, theentire amount of NOx released and reduced in the main NOx catalyst 55.On the other hand, the second release counter CC2 is formed by a storageregion set in a predetermined region of the RAM 33, a register containedin the CPU 34 or the like, and retains the integrated value of theamount of NOx released and reduced in the sub NOx catalyst 61, that is,the entire amount of NOx released and reduced in the sub NOx catalyst61.

(Step 908)

The ECU 30 reads the counter value :CC1 of the first release counter CC1updated in Step 907, and reads the counter value :C1 of theabove-described first absorption counter C1, and determines whether thecounter value :CC1 of the first release counter CC1 is not smaller thanthe counter value :C1 of the above-described first absorption counterC1.

(Step 909)

When determined affirmative in Step 908, the ECU 30 proceeds to Step909, and resets the counter value :C1 of the first absorption counter C1to “0”.

(Step 910)

When determined negative in Step 908, the ECU 30 proceeds to Step 910,and the value (C1-CC1) obtained by subtracting the counter value :CC1 ofthe first release counter CC1 from the counter value :C1 of the firstabsorption counter C1, is regarded as the new counter value of the firstabsorption counter C1.

(Step 911)

After the execution of the processing of Step 909 or Step 910, the ECU30 proceeds to Step 911. In Step 911, the ECU 30 resets the countervalue :CC1 of the first release counter CC1 to “0”.

(Step 912)

The ECU 30 reads the counter value :CC2 of the second release counterCC2 updated in Step 907, and reads the counter value :C2 of the secondrelease counter C2, and makes a determination as to whether the countervalue :CC2 of the second release counter CC2 is higher than the countervalue :C2 of the second absorption counter C2.

(Step 913)

When determined affirmative in Step 912, the ECU 30 proceeds to Step913, and resets the counter value :C2 of the second absorption counterC2 to “0”.

(Step 914)

When determined negative in Step 912, the ECU 30 proceeds to Step 914,and the value (C2-CC2) obtained by subtracting the second counter value:CC2 of the second release counter C2 from the counter value :C2 of thesecond absorption counter C2, is regarded as the new counter value ofthe second absorption counter C2.

(Step 915)

After the execution of the processing of Step 913 or Step 914, the ECU30 proceeds to Step 915. In Step 915, the ECU 30 resets the countervalue :CC2 of the second release counter CC2 to “0”. After the executionof the processing of Step 915, the ECU 30 temporarily terminates theexecution of the routine.

(Step 916)

On the other hand, when determined negative in Step 905, the ECU 30proceeds to Step 916. In Step 916, the ECU 30 changes the value of thefirst NOx release flag storage region from “0” to “1”.

(Step 917)

When the execution of the processing of Step 916 is completed, or whendetermined negative in Step 901, the ECU 30 proceeds to Step 917. InStep 917, the ECU 30 executes the rich spike control on the main NOxcatalyst 55. More specifically, the ECU 30 controls the first actuator62 and the second actuator 64 so as to retain the first exhaustswitching valve 63 in the fully open state and retain the second exhaustswitching valve 65 in the totally closed state, and switches theoperating condition of the internal combustion engine 1 to the richair-fuel ratio operation.

(Step 918)

In Step 918, the ECU 30 calculates the amount of NOx released andreduced in the main NOx catalyst 55 on the basis of the amount ofunburned fuel components discharged from the internal combustion engine1, and updates the counter value :CC1 of the first release counter CC1on the basis of the NOx amount calculated.

(Step 919)

In Step 919, the ECU 30 reads the counter value :CC1 of the firstrelease counter CC1 updated in Step 918, and reads the counter value :C1of the first absorption counter C1, and determines whether the countervalue :CC1 of the first release counter CC1 is higher than the countervalue :C1 of the first absorption counter C1.

When determined negative in Step 919, the ECU 30 returns to Step 917,and continues the rich spike control to the main NOx catalyst 55. On theother hand, when deternubed affirmative in Step 919, the ECU 30 proceedsto Step 920.

(Step 920)

In Step 920, the ECU 30 terminates the execution of rich spike controlto the main NOx catalyst 55. More specifically, the ECU 30 returns thecontrol of the first exhaust switching valve 63 and the second exhaustswitching valve 65 and the control of the internal combustion engine 1to normal. Subsequently, the ECU 30 changes the value of the firstrelease flag storage region from “1” to “0”, and resets the countervalues of the first absorption counter C1 and the first release counterCC1 to “0”. After the execution of the processing of Step 920, the ECU30 temporarily terminates the execution of the routine.

(Step 921)

Further, when determined negative in Step 906, the ECU 30 proceeds toStep 921, and changes the value of the second release flag storageregion from “0” to “1”.

(Step 922)

When the execution of the processing of Step 921 is completed, or whendetermined negative in Step 902, the ECU 30 proceeds to Step 922. InStep 922, the ECU 30 executes the rich spike control to the sub NOxcatalyst 61. More specifically, the ECU controls the first actuator 62and the second actuator 64 so as to retain the first exhaust switchingvalve 63 in the totally closed state and the second exhaust switchingvalve 65 in the fully open state, and switches the operating conditionof the internal combustion engine 1 to the rich air-fuel ratiooperation.

(Step 923)

In Step 923, the ECU 30 updates the counter value :CC2 of the secondrelease counter CC2 on the basis of the amount of unburned fuelcomponents discharged from the internal combustion engine 1.

(Step 924)

In Step 924, the ECU 30 reads the counter value :CC2 of the secondrelease counter CC2 updated in Step 923, and reads the counter value :C2of the second absorption counter C2, and determines whether the countervalue :CC2 of the second release counter CC2 is higher than the countervalue :C2 of the second absorption counter C2.

When determined negative in Step 924, the ECU 30 returns to Step 922,and continues the rich spike control on the sub NOx catalyst 61. On theother hand, when determined affirmative in Step 924, the process by theECU 30 advances to Step 925.

(Step 925)

In Step 925, the ECU 30 terminates the execution of the rich spikecontrol on the sub NOx catalyst 61. More specifically, the ECU 30returns the control of the first exhaust switching valve 63 and thesecond exhaust switching valve 65 and the control of the internalcombustion engine 1 to normal. Subsequently, the ECU 30 changes thevalue of the second NOx release flag storage region from “1” to “0”, andresets the values of the second absorption counter C2 and the secondrelease counter CC2 to “0”. After the execution of the processing ofStep 925, the ECU 30 temporarily terminates the execution of theroutine.

In the eighth embodiment described above, the NOx absorption amount ofthe main NOx catalyst 55 is estimated taking into account the amount ofexhaust leaking from the first exhaust switching valve 63 and the secondexhaust switching valve 65, so that it is possible to accuratelyestimate the NOx absorption amount of the main NOx catalyst 55, wherebyit is possible to execute the rich spike control with high accuracy onthe main NOx catalyst 55.

Further, in the eighth embodiment, the NOx absorption amount of the subNOx catalyst 61 is estimated taking into account the amount of exhaustleaking from the first exhaust switching valve 63 and the second exhaustswitching valve 65, and it is possible to execute the rich spike controlon the sub NOx catalyst 61 on the basis of the estimated value, so thatit is possible to reliably reduce the NOx inadvertently absorbed by thesub NOx catalyst 61, thereby making it possible to achieve animprovement in exhaust emission control.

In the eighth embodiment described above, the NOx absorption amounts ofthe main NOx catalyst 55 and the sub NOx catalyst 61 are estimatedtaking into account the amount of exhaust leaking from the first exhaustswitching valve 63 and the second exhaust switching valve 65. In somecases, however, there is a response delay of the first exhaust switchingvalve 63 and the second exhaust switching valve 65, that is, it takessome time from the point when the first actuator 62 or the secondactuator 64 is controlled so as to switch the first exhaust switchingvalve 63 or the second exhaust switching valve 65 from the fully openstate to the totally closed state (or from the totally closed state tothe fully open state) to the point when the first exhaust switchingvalve 63 or the second exhaust switching valve 65 is actually broughtinto the totally closed state (or fully open state) . In such a case, itis desirable to estimate the NOx absorption amount taking into accountthe amount of exhaust flowing through the main NOx catalyst 55 or thesub NOx catalyst 61 during the response delay period of the firstexhaust switching valve 63 and the second exhaust switching valve 65.

Ninth Embodiment

Next, a ninth embodiment of the exhaust emission purifying device for aninternal combustion engine of the present invention will be described.Here, the construction differing from that of the eighth embodiment isdescribed, and a description of the construction similar to that of theeighth embodiment will be omitted.

The ninth embodiment differs from the eighth embodiment in the followingpoint. In the eighth embodiment, the rich spike control on the main NOxcatalyst 55 and the rich spike control on the sub NOx catalyst 61 areexecuted independently of each other. In the ninth embodiment, incontrast, the rich spike control on the main NOx catalyst 55 and therich spike control on the sub NOx catalyst 61 are executed insynchronism with each other only when the NOx catalyst temperature riserestraining control is executed, in other words, only when the exhaustpurification is effected by using both the main NOx catalyst 55 and thesub NOx catalyst 61.

This is because when, at the time of execution of the NOx catalysttemperature rise restraining control, the execution time of the richspike control on the main NOx catalyst 55 differs from the executiontime of the rich spike control on the sub NOx catalyst 61, it is to beexpected that the execution frequency of the rich spike control willincrease, resulting in an increase in the fuel consumption amount.

However, since it may happen that at the start of the execution of theNOx catalyst temperature rise restraining control the NOx absorptionamount of the main NOx catalyst 55 differs from the NOx absorptionamount of the sub NOx catalyst 61, the ninth embodiment adopts anarrangement in which immediately before the execution of the NOxcatalyst temperature rise restraining control, all the NOx absorbed bythe main NOx catalyst 55 and the sub NOx catalyst 61 is temporarilyreleased and reduced.

Further, when the limit value of the amount of NOx that can be absorbedby the main NOx catalyst 55 (hereinafter referred to as the first NOxabsorption limit value) differs from the limit value of the amount ofNOx that can be absorbed by the sub NOx catalyst 61 (hereinafterreferred to as the second NOx absorption limit value), even if all theNOx absorbed by the main NOx catalyst 55 and the sub NOx catalyst 61 isreleased and reduced immediately before the execution of the NOxcatalyst temperature rise restraining control, the timing with which theNOx absorbing capacity of the main NOx catalyst 55 is saturated isdeviated from the timing with which the Nox absorbing capacity of thesub NOx catalyst 61 is saturated, making it impossible to synchronizethe execution times of the rich spike control. Thus, in the ninthembodiment, the rich spike control is executed by using, as a reference,of the main NOx catalyst 55 and the sub NOx catalyst 61, the one whoseNOx absorbing capacity is less.

In the example described below, lean/rich spike control is conductedwhen the NOx absorbing capacity of the main NOx catalyst 55 is higherthan the NOx absorbing capacity of the sub NOx catalyst 61, that is,when the first NOx absorption limit value is higher than the second NOxabsorption limit value.

In the ninth embodiment, when executing lean/rich spike control, the ECU30 executes lean/rich spike control in accordance with the lean/richspike control routine shown in FIG. 17. The lean/rich spike controlroutine shown in FIG. 17 is stored in advance in the ROM 32 of the ECU30, and is a routine repeatedly executed by the CPU 34 for eachpredetermined time.

(Step 1001)

In the lean/rich spike control routine, the ECU 30 first determines inStep 1001 whether the exhaust is in the condition in which it is to becaused to flow through both the main NOx catalyst 55 and the sub NOxcatalyst 61, in other words, whether the engine operating condition isin the lean/rich spike control execution range and the exhausttemperature is higher than a predetermined temperature.

(Step 1019)

When determined negative in Step 1001, that is, when the exhaust is notin the condition in which it is to be caused to flow through both themain NOx catalyst 55 and the sub NOx catalyst 61, the ECU 30 proceeds toStep 1019, in which the normal lean/rich spike control is executed.Here, the normal lean/rich spike control is the same as the lean/richspike control described with reference to the eighth embodiment.

(Step 1002)

When determined affirmative in Step 1001, that is, when the exhaust isin the condition in which it is to be caused to flow through both themain NOx catalyst 55 and the sub NOx catalyst 61, the ECU 30 proceeds toStep 1002, in which a determination is made as to whether the countervalue :C1 of the first absorption counter C1 is larger than “0”, thatis, whether NOx has been absorbed by the main NOx catalyst 55.

(Step 1003)

When determined affirmative in Step 1002, the process by the ECU 30advances to Step 1003, and executes the rich spike control so as torelease and reduce all the NOx absorbed by the main NOx catalyst 55.More specifically, the ECU 30 controls the first actuator 62 and thesecond actuator 64 so as to retain the first exhaust switching valve 63in the fully open state and retain the second exhaust switching valve 65in the totally closed state, and switches the engine operating conditionto the rich air-fuel ratio operation.

(Step 1004)

In Step 1004, the ECU calculates the amount of NOx released and reducedby the main NOx catalyst 55 on the basis of the amount of unburned fuelcomponents discharged from the internal combustion engine 1, and updatesthe counter value :CC1 of the first release counter CC1 on the basis ofthe calculated NOx amount.

(Step 1005)

In Step 1005, the ECU 30 reads the counter value :CC1 of the firstrelease counter CC1 updated in Step 1004, and reads the counter value C1of the first absorption counter C1, making a determination as to whetherthe counter value :CC1 of the first release counter CC1 is higher thanthe counter value :C1 of the first absorption counter C1.

When determined negative in Step 1005, the ECU 30 returns to Step 1003,and continues the execution of the rich spike control on the main NOxcatalyst 55. On the other hand, when determined affirmative in Step1005, the ECU 30 proceeds to Step 1006.

(Step 1006)

In Step 1006, the ECU 30 determines whether the counter value :C2 of thesecond absorption counter C2 is larger than “0”, that is, whether NOxhas been absorbed by the sub NOx catalyst 61.

(Step 1007)

When determined affirmative in Step 1006, the ECU 30 proceeds to Step1007, and executes the rich spike control so as to release and reduceall the NOx absorbed by the sub NOx catalyst 61. More specifically, theECU 30 controls the first actuator 62 and the second actuator 64 so asto retain the first exhaust switching valve 63 in the totally closedstate and retain the second exhaust switching valve 65 in the fully openstate, and switches the engine operating condition to the rich air-fuelratio operation.

(Step 1008)

In Step 1008, the ECU calculates the amount of NOx released and reducedby the sub NOx catalyst 61 on the basis of the amount of unburned fuelcomponents discharged from the internal combustion engine 1, and updatesthe counter value :CC2 of the second release counter CC2 on the basis ofthe NOx amount calculated.

(Step 1009)

In Step 1009, the ECU 30 reads the counter value :CC2 of the secondrelease counter CC2 updated in Step 1008, and reads the counter value:C2 of the second absorption counter C2, and determines whether thecounter value :CC2 of the second release counter CC2 is higher than thecounter value :C2 of the second absorption counter C2.

When determined negative in Step 1009, the ECU 30 returns to Step 1007,and continues the rich spike control on the sub NOx catalyst 61. On theother hand, when determined affirmative in Step 1009, the ECU 30proceeds to Step 1010.

(Step 1010)

In Step 1010, the ECU 30 controls the first actuator 62 and the secondactuator 64 so as to retain both the first exhaust switching valve 63and the second exhaust switching valve 65 in the fully open state, andswitches the engine operating condition to the lean air-fuel ratiooperation.

(Step 1011)

In Step 1011, the ECU 30 gains access to a third NOx release flagstorage region pre-set in a predetermined region of the RAM 33, anddetermines whether “1” is stored or not.

The third NOx release flag storage region is a region which stores “1”when the amount of NOx absorbed by the main NOx catalyst 55 and the subNOx catalyst 61 is higher than double the second NOx absorption limitvalue (double the first NOx absorption limit value when the first NOxabsorption limit value>the second NOx absorption limit value), andstores “0” when the amount of NOx absorbed by the main NOx catalyst 55and the sub NOx catalyst 61 is less than double the second NOxabsorption limit value.

(Step 1012)

When determined afirmative in Step 1011, that is, when it is determinedthat “0” is stored in the third NOx release flag storage region of theRAM 33, the ECU 30 proceeds to Step 1012. In Step 1012, the ECU 30calculates the engine exhaust NOx amount by using the engine speed, fuelinjection amount, etc. as parameters, and adds the engine exhaust NOxamount to a counter value :C3 of a third absorption counter C3.

The third absorption counter C3 is formed by a storage region set in apredetermined region of the RAM 33, a register contained in the CPU 34,etc., and retains the integrated value of the amount of NOx absorbed bythe main NOx catalyst 55 and the sub NOx catalyst 61, in other words,the entire amount of NOx absorbed by the main NOx catalyst 55 and thesub NOx catalyst 61.

(Step 1013)

The ECU 30 reads the counter value :C3 of the third absorption counterC3 updated in Step 1013, and compares the counter value :C3 with doublethe second NOx absorption limit value :C3MAX. More specifically, the ECU30 determines whether the counter value :C3 is higher than double thesecond NOx absorption limit value :C3MAX.

When determined negative in Step 1013, the ECU 30 returns to Step 1012.On the other hand, when determined affirmative in Step 1013, the ECU 30proceeds to Step 1014.

(Step 1014)

In Step 1014, the ECU 30 changes the value of the third release flagstorage region from “0” to “1”.

Here, when determined affirmative in Step 1011, or when the ECU 30 hascompleted the execution of the processing of Step 1014, the ECU 30proceeds to Step 1015.

(Step 1015)

In Step 1015, the ECU 30 switches the engine operating condition fromthe lean air-fuel ratio operation to the rich air-fuel ratio operation,whereby the exhaust of rich air-fuel ratio is caused to flow throughboth the main NOx catalyst 55 and the sub NOx catalyst 61, therebyreleasing and reducing the NOx absorbed by the main NOx catalyst 55 andthe sub NOx catalyst 61.

(Step 1016)

In Step 1016, the ECU 30 calculates the amount of NOx released andreduced by the main NOx catalyst 55 and the sub NOx catalyst 61 on thebasis of the amount of unburned fuel components discharged from theinternal combustion engine 1, and updates the counter value :CC3 of thethird release counter CC3 on the basis of the NOx amount calculated.

The third release counter CC3 is formed of a storage region set in apredetermined region of the RAM 33, a register contained in the CPU 34or the like, and retains the integrated value of NOx released andreduced by the main NOx catalyst 55 and the sub NOx catalyst 61, inother words, the entire amount of NOx released and reduced by the mainNOx catalyst 55 and the sub NOx catalyst 61.

(Step 1017)

In Step 1017, the ECU 30 reads the counter value :CC3 of the thirdrelease counter CC3 updated in Step 1016, and the counter value :C3 ofthe third absorption counter C3, and makes a determination as to whetherthe counter value :CC3 of the third release counter CC3 is higher thanthe counter value :C3 of the third absorption counter C3, that is,whether all the NOx absorbed by the main NOx catalyst 55 and the sub NOxcatalyst 61 has been released or cleaned.

When determined negative in Step 1017, the ECU returns to Step 1015, andcontinues the rich spike control on the main NOx catalyst 55 and the subNOx catalyst 61. On the other hand, when determined affirmative in Step1017, the ECU 30 proceeds to Step 1018.

(Step 1018)

In Step 1018, the ECU 30 terminates the execution of the rich spikecontrol on the main NOx catalyst 55 and the sub NOx catalyst 61. Morespecifically, the ECU 30 switches the engine operating condition fromthe rich air-fuel ratio operation to the lean air-fuel ratio operation.Further, the ECU 30 changes the value of the third NOx release flagstorage region from “1” to “0”, and resets the counter value :C3 of thethird absorption counter C3 and the counter value :CC3 of the thirdrelease counter CC3 to “0”. After the execution of the processing ofStep 1018, the ECU 30 temporarily terminates the execution of theroutine.

In the ninth embodiment described above, when the exhaust flows throughboth the main NOx catalyst 55 and the sub NOx catalyst 61, it ispossible to execute the rich spike control on the main NOx catalyst 55in synchronism with the rich spike control on the sub NOx catalyst 61,so that the execution frequency of the rich spike control decreases,with the result that it is possible to reduce the fuel consumptionamount related to the rich spike control.

Tenth Embodiment

Next, a tenth embodiment of the exhaust emission purifying device for aninternal combustion engine of the present invention will be described.Here, the construction differing from that of the fourth embodimentdescribed above will be described, and a description of the constructionsimilar to that of the fourth embodiment will be omitted.

The tenth embodiment differs from the fourth embodiment in that in themain NOx catalyst temperature rise control in the fourth embodiment themain NOx catalyst 55 is activated after the completion of the warming upof the internal combustion engine 1, while in the tenth embodiment, themain NOx catalyst 55 is activated when the internal combustion engine 1is warmed up.

In the fourth embodiment, the engine warming-up control, that is thestoichiometric operation of the internal combustion engine is continueduntil the amount of NOx discharged from the internal combustion engine 1becomes smaller than a predetermined amount after the completion of thewarming up of the internal combustion engine, so that when the time fromthe point when the warming up of the internal combustion engine 1 iscompleted to the point when the amount of NOx discharged from theinternal combustion engine 1 becomes smaller than a predetermined amountis rather long, it is to be expected that the fuel consumption amountwill be increased.

In view of this, in the tenth embodiment, the opening and closing of thefirst exhaust switching valve 63 and the second exhaust switching valve65 are controlled such that, in the warming-operation of the internalcombustion engine 1, while the air-fuel ratio of the exhaust isstoichiometric, the entire amount of exhaust flows through the sub NOxcatalyst 61, and while the amount of NOx in the exhaust is smaller thana predetermined amount, the entire amount of exhaust flows through themain NOx catalyst 55, whereby the warming up of the internal combustionengine 1 and the activation of the main NOx catalyst 55 are conducted inparallel.

Examples of the amount of NOx in the exhaust is smaller than apredetermined amount are that, when the vehicle is running at reducedspeed, when the execution of the fuel injection control is inhibited,and the execution of sparking control is inhibited. The tenth embodimentwill be described with reference to the case in which the vehicle isrunning at reduced speed.

In the following, the main NOx catalyst temperature rise control will bedescribed with reference to the flowchart of FIG. 18. The flowchart ofFIG. 18 shows a main NOx catalyst temperature rise control routine. Thismain NOx catalyst temperature rise control routine is stored in advancein the ROM 32 of the ECU 30, and is a routine executed by the CPU 34,using the starting completion of the internal combustion engine 1.

(Step 1101)

In the main NOx catalyst temperature rise control routine, the ECU 30first determines in Step 1101 hether the starting of the internalcombustion engine 1 has been completed or not.

When determined negative in Step 1101, the ECU 30 executes theprocessing of Step 1101 again. On the other hand, when determinedaffirmative in Step 1101, the ECU 30 proceeds to Step 1102.

(Step 1102)

In Step 1102, the ECU 30 executes the engine warming-up processing. Morespecifically, the ECU 30 operates the internal combustion engine 1 atthe stoichiometric air-fuel ratio, and controls the first actuator 62and the second actuator 64 so as to retain the first exhaust switchingvalve 63 in the totally closed state and retain the second exhaustswitching valve 65 in the fully open state.

In this case, the exhaust of stoichiometric air-fuel ratio is dischargedfrom the internal combustion engine 1, and the harmful gas componentscontained in the exhaust, such as HC, CO, and NOx, are purified by the3-way catalyst 51 activated by the 3-way catalyst temperature risecontrol at the time of engine start-up. The exhaust of which the harmfulgas components have been purified by the 3-way catalyst 51 is led to theexhaust pipe 58 by way of the sub NOx catalyst 61 in the bypass path 59.In the process, the sub NOx catalyst 61 is not activated yet. However,since the harmful gas components in the exhaust have been purified bythe 3-way catalyst 51 as stated above, there is no deterioration inexhaust emission. Further, the sub NOx catalyst 61 receives heat fromthe exhaust and undergoes temperature rise.

(Step 1103)

In Step 1103, the ECU 30 determines whether the vehicle is running atdecelerated speed or not. An example of a method for determining therunning of the vehicle at decelerated speed is that it is determinedunder the conditions that the vehicle is running at deceleerated speedwhen the operating amount of the accelerator (not shown) is “zero” andthe vehicle speed is higher than a predetermined speed.

(Step 1104)

When determined affirmative in Step 1103, the ECU 30 determines that theamount of NOx in the exhaust is less than a predetermined amount, andthe temperature rise processing is executed on the main NOx catalyst 55.More specifically, the ECU 30 controls the first actuator 62 so as toswitch the first exhaust switching valve 63 from the totally closedstate to the fully open state, and controls the second actuator 64 so asto switch the second exhaust switching valve 65 from the fully openstate to the totally closed state, causing the entire amount of exhaustto flow through the main NOx catalyst 55.

In the process, the exhaust discharged from the internal combustionengine 1 flows to the exhaust pipe 58 by way of the main NOx catalyst55. However, since the amount of NOx contained in the exhaust dischargedfrom the internal combustion engine 1 during the decelerated-speedrunning is very small, there is no rapid deterioration in exhaustemission even when the main NOx catalyst 55 is not activated yet.

Further, even when no combustion is conducted in the internal combustionengine 1, the exhaust discharged from the internal combustion engine 1when the vehicle is running at decelerated speed receives heat from theinterior of the engine, so that when such an exhaust passes through themain NOx catalyst 55, the main NOx catalyst 55 receives heat from theexhaust and undergoes temperature rise.

After the execution of the processing of Step 1104 as described above,the ECU 30 returns to Step 1103, in which a determination is made as towhether the running state of the vehicle at decelerted speed is beingcontinued or not. When determined affirmative in Step 1103, that is,when it is determined that the running of the vehicle at deceleratedspeed is being continued, the ECU 30 proceeds to Step 1104, andcontinues the temperature raising processing on the main NOx catalyst55. On the other hand, when determined negative in Step 1103, that is,when it is determined that the running state of the vehicle atdecelerated speed has been completed, the ECU 30 proceeds to Step 1105.

(Step 1105)

In Step 1105, the ECU 30 determines whether the warming up of theinternal combustion engine 1 has been completed or not. Examples of amethod for determining the completion of the warming up of the internalcombustion engine 1 include one in which it is determined that thewarming up of the internal combustion engine 1 has been completed whenthe temperature of the engine cooling water is higher than apredetermined temperature, and one in which a determination as towhether the warming up of the internal combustion engine 1 (and theactivation of the sub NOx catalyst 61) has been completed or not, usingthe operation history since the starting of the internal combustionengine 1, etc. as parameters.

When determined negative in Step 1105, the processs by the ECU 30returns to Step 1102, and continues the execution of the engine warmingup processing. On the other hand, when determined affirmative in Step1105, the ECU 30 proceeds to Step 1106.

(Step 1106)

In Step 1106, the ECU 30 terminates the execution of engine warming upprocessing. More specifically, the ECU 30 switches the operatingcondition of the internal combustion engine 1 from the stoichiometricoperation to the lean/rich spike operation, and controls the firstactuator 62 and the second actuator 64 so as to switch the first exhaustswitching valve 63 from the totally closed state to the fully open stateand switch the second exhaust switching valve 65 from the fully openstate to the totally closed state. After the execution of the processingof Step 1106, the ECU 30 terminates the execution of the routine.

As described above, in the exhausted mission purifying device of thetenth embodiment, it is possible to raise the temperature of the mainNOx catalyst 55 without involving a deterioration in exhaust emissionduring the warming up operation of the internal combustion engine 1, sothat it is possible to activate the NOx catalyst 55 while suppressingthe execution range of the engine warming up control to a minimum.

When causing the entire amount of exhaust to flow through the main NOxcatalyst 55 during the running of the vehicle at reduced speed, it isalso possible to increase the degree of opening of the throttle valve15, thereby increasing the quantity of heat transmitted from the exhaustto the main NOx catalyst 55. Alternately, it is also possible tosecondarily inject the fuel from the fuel injection valve 11 to burn thefuel at the 3-way catalyst 51 to thereby raise the temperature of theexhaust, thereby increasing the quantity of heat transmitted from theexhaust to the main NOx catalyst 55.

Other Embodiments

In the third to tenth embodiments, the exhaust emission purifying devicefor an internal combustion engine of the present invention is describedin which the sub NOx catalyst 61 is arranged in the bypass path 59bypassing the main NOx catalyst 55, that is, the main NOx catalyst 55and the sub NOx catalyst 61 are arranged in parallel. It is alsopossible, as shown in FIG. 19, to arrange the main NOx catalyst 55 andthe sub NOx catalyst 61 in series in an exhaust passage 70 such that themain NOx catalyst 55 is situated on the upstream than the sub NOxcatalyst 61, wherein there are provided a bypass path 71 communicatingan exhaust passage 70 which is on the upstream side of the main NOxcatalyst 55 with the exhaust passage 70 which is on the upstream side ofthe sub NOx catalyst 61 and on the downstream side of the main NOxcatalyst 55, and an exhaust switching valve 72 provided in the branchingportion between the bypass path 71 and the main NOx catalyst 55 andadapted to switch the exhaust flow between the bypass path 71 and themain NOx catalyst 55.

Further, while in the first to tenth embodiments described above thepresent invention is applied to a gasoline engine, the present inventionis also applicable to a diesel engine. In the case of a diesel engine,the combustion in the combustion chamber is effected at an air-fuelratio which is much higher than the stoichiometric air-fuel ratio, sothat, in the normal engine operating condition, the exhaust flowing intothe SOx absorbing material 17 and the main NOx catalyst 20 is very lean,and, while the absorption of SOx and NOx is effected, little or noreleasing of SOx and NOx is effected.

Further, as described above, in the case of a gasoline engine, it ispossible to make the air fuel ratio of the exhaust flowing. into the SOxabsorbing material 17 and the main NOx catalyst 20 stoichiometric orrich by making the air-fuel ratio of the air-fuel mixture supplied tothe combustion chamber 3 stoichiometric or rich, and to release the SOxand NOx absorbed by the SOx absorbing material 17 and the main NOxcatalyst 20. In the case of a diesel engine, making the air-fuel ratioof the air-fuel mixture supplied to the combustion engine stoichiometricor rich leads to the generation of soot, etc. at the time of combustion,so that this method cannot be adopted.

Thus, when applying the present invention to a diesel engine, to makethe air-fuel ratio of the exhaust flowing in stoichiometric or rich, itis necessary to supply a reducing agent (e.g., diesel oil serving asfuel) to the exhaust apart from the burning of the fuel for obtainingengine output. The supply of a reducing agent to the exhaust can beeffected through the secondary injection of fuel into the cylinderduring the intake stroke, expansion stroke, or exhaust stroke, or bysupplying the reducing agent to the exhaust passage on the upstream sideof the SOx absorbing material 17.

If the diesel engine is provided with an exhaust recirculating device(so-called EGR device), it is possible to make the air-fuel ratio of theexhaust stoichiometric or rich by introducing a large amount of exhaustrecirculation gas into the combustion chamber.

What is claimed is:
 1. An exhaust emission purifying device of aninternal combustion engine, comprising: a lean-burn type internalcombustion engine capable of burning an air-fuel mixture with excessiveoxygen; an NOx absorbing material which is arranged in an exhaustpassage of the internal combustion engine and which is adapted to absorbnitrogen oxides in the exhaust when an air-fuel ratio of the exhaustflowing is lean and to release the nitrogen oxides it has absorbed whenthe oxygen concentration of the exhaust flowing in is low; a bypass pathbranching off from the portion of the exhaust passage on the upstreamside of the NOx absorbing material and adapted to cause the exhaust tobypass the NOx absorbing material; an exhaust flow switching means forselectively switching the flow of the exhaust between the NOx absorbingmaterial and the bypass path; an SOx absorbing material which isarranged in the portion of the exhaust passage on the upstream side ofthe exhaust flow switching means and which is adapted to absorb sulfuroxides when the air-fuel ratio of the exhaust flowing-in is lean and torelease the sulfur oxides it has absorbed when the oxygen concentrationof the exhaust flowing in is low; and an NOx catalyst provided in thebypass path and adapted to purify nitrogen oxides when the air-fuelratio of the exhaust flowing in is lean; wherein the exhaust flowswitching means causes all the exhaust to flow through the NOx absorbingmaterial when the air-fuel ratio of the exhaust is controlled to belean, and causes all the exhaust to flow through the bypass path whenthe air-fuel ratio of the exhaust is controlled to be stoichiometric orrich.
 2. An exhaust emission purifying device of an internal combustionengine according to claim 1, wherein the NOx catalyst provided in thebypass path is a selective reduction type NOx catalyst adapted to reduceor decompose nitrogen oxides in an oxygen-excessive atmosphere in thepresence of hydrocarbon.
 3. An exhaust emission purifying device of aninternal combustion engine according to claim 1, wherein the NOxcatalyst provided in the bypass path is an occlusion reduction type NOxcatalyst which absorbs nitrogen oxides in the exhaust when the air-fuelratio of the exhaust is lean and which releases the nitrogen oxides ithas absorbed to reduce or decompose them when the oxygen concentrationof the exhaust is less and there exists a reducing agent.
 4. An exhaustemission purifying device of an internal combustion engine according toclaim 1, further comprising a temperature rise restraining means forcontrolling the exhaust flow switching means so as to cause the exhaustto flow through both the NOx absorbing material and the NOx catalystwhen the temperature of the NOx absorbing material becomes higher than apredetermined temperature when the exhaust flow switching means is beingcontrolled to cause all the exhaust to flow through the NOx absorbingmaterial.
 5. An exhaust emission purifying device of an internalcombustion engine according to claim 3, further comprising an NOxabsorption amount detection means for detecting the amount of NOxabsorbed by the NOx absorbing material and the amount of NOx absorbed bythe NOx catalyst.
 6. An exhaust emission purifying device of an internalcombustion engine, comprising: a lean-bum type internal combustionengine capable of burning an air-fuel mixture with excessive oxygen; anNOx absorbing material which is arranged in an exhaust passage of theinternal combustion engine and which is adapted to absorb nitrogenoxides in the exhaust when an air-fuel ratio of the exhaust flowing islean and to release the nitrogen oxides it has absorbed when the oxygenconcentration of the exhaust flowing in is low; a bypass path branchingoff from the portion of the exhaust passage on the upstream side of theNOx absorbing material and adapted to cause the exhaust to bypass theNOx absorbing material; an exhaust flow switching means for selectivelyswitching the flow of the exhaust between the NOx absorbing material andthe bypass path; an SOx absorbing material which is arranged in theportion of the exhaust passage on the upstream side of the exhaust flowswitching means and which is adapted to absorb sulfur oxides when theair-fuel ratio of the exhaust flowing-in is lean and to release thesulfur oxides it has absorbed when the oxygen concentration of theexhaust flowing in is low; and an NOx catalyst provided in the bypasspath and adapted to purify nitrogen oxides when the air-fuel ratio ofthe exhaust flowing in is lean, wherein the NOx catalyst provided in thebypass path has a 3-way purifying function and HC adsorbing capacity atlow temperature, and wherein the exhaust flow switching means causes allthe exhaust to flow through the bypass path when the temperature of theexhaust is less than a predetermined temperature, and causes all theexhaust to flow through the NOx absorbing material when the temperatureof the exhaust is higher than the predetermined temperature.
 7. Anexhaust emission purifying device of an internal combustion engine,comprising: a lean-burn type internal combustion engine capable ofburning an air-fuel mixture with excessive oxygen; an NOx absorbingmaterial which is arranged in an exhaust passage of the internalcombustion engine and which is adapted to absorb nitrogen oxides in theexhaust when an air-fuel ratio of the exhaust flowing is lean and torelease the nitrogen oxides it has absorbed when the oxygenconcentration of the exhaust flowing in is low; a bypass path branchingoff from the portion of the exhaust passage on the upstream side of theNOx absorbing material and adapted to cause the exhaust to bypass theNOx absorbing material; an exhaust flow switching means for selectivelyswitching the flow of the exhaust between the NOx absorbing material andthe bypass path; an SOx absorbing material which is arranged in theportion of the exhaust passage on the upstream side of the exhaust flowswitching means and which is adapted to absorb sulfur oxides when theair-fuel ratio of the exhaust flowing-in is lean and to release thesulfur oxides it has absorbed when the oxygen concentration of theexhaust flowing in is low; and an NOx catalyst provided in the bypasspath and adapted to purify nitrogen oxides when the air-fuel ratio ofthe exhaust flowing in is lean, wherein when the internal combustionengine is an in-cylinder injection type internal combustion engine whichis provided with a fuel injection vale for directly injecting the fuelin a combustion chamber of the internal combustion engine, wherein theSOx absorbing material has a 3-way function, and wherein, when startingthe internal combustion engine, the exhaust flow switching means iscontrolled so as to throttle the amount of exhaust flowing through theNOx absorbing material and the NOx catalyst, and the fuel injectionvalve is controlled so as to secondarily inject fuel during theexpansion stroke of each cylinder in addition to the injection of thefuel to be burnt.
 8. An exhaust emission purifying device of an internalcombustion engine, comprising: a lean-bum type internal combustionengine capable of burning an air-fuel mixture with excessive oxygen; anNOx absorbing material which is arranged in an exhaust passage of theinternal combustion engine and which is adapted to absorb nitrogenoxides in the exhaust when an air-fuel ratio of the exhaust flowing islean and to release the nitrogen oxides it has absorbed when the oxygenconcentration of the exhaust flowing in is low; a bypass path branchingoff from the portion of the exhaust passage on the upstream side of theNOx absorbing material and adapted to cause the exhaust to bypass theNOx absorbing material; an exhaust flow switching means for selectivelyswitching the flow of the exhaust between the NOx absorbing material andthe bypass path; an SOx absorbing material which is arranged in theportion of the exhaust passage on the upstream side of the exhaust flowswitching means and which is adapted to absorb sulfur oxides when theair-fuel ratio of the exhaust flowing-in is lean and to release thesulfur oxides it has absorbed when the oxygen concentration of theexhaust flowing in is low; and an NOx catalyst provided in the bypasspath and adapted to purify nitrogen oxides when the air-fuel ratio ofthe exhaust flowing in is lean, wherein, when the temperature of the NOxabsorbing material becomes higher than a predetermined temperature whenthe exhaust flow switching means is being controlled so as to cause allthe exhaust to flow through the NOx absorbing material, a temperaturerise restraining means controls the exhaust flow switching means so asto cause the exhaust to flow through both the NOx absorbing material andthe NOx catalyst after executing an SOx poisoning regenerativeprocessing on the NOx catalyst.
 9. An exhaust emission purifying deviceof an internal combustion engine, comprising: a lean-bum type internalcombustion engine capable of burning an air-fuel mixture with excessiveoxygen; an NOx absorbing material which is arranged in an exhaustpassage of the internal combustion engine and which is adapted to absorbnitrogen oxides in the exhaust when an air-fuel ratio of the exhaustflowing is lean and to release the nitrogen oxides it has absorbed whenthe oxygen concentration of the exhaust flowing in is low; a bypass pathbranching off from the portion of the exhaust passage on the upstreamside of the NOx absorbing material and adapted to cause the exhaust tobypass the NOx absorbing material; an exhaust flow switching means forselectively switching the flow of the exhaust between the NOx absorbingmaterial and the bypass path; an SOx absorbing material which isarranged in the portion of the exhaust passage on the upstream side ofthe exhaust flow switching means and which is adapted to absorb sulfuroxides when the air-fuel ratio of the exhaust flowing-in is lean and torelease the sulfur oxides it has absorbed when the oxygen concentrationof the exhaust flowing in is low; and an NOx catalyst provided in thebypass path and adapted to purify nitrogen oxides when the air-fuelratio of the exhaust flowing in is lean, wherein, when the internalcombustion engine is performing warming-up operation, the exhaust flowswitching means is controlled so as to cause all the exhaust to flowthrough the NOx catalyst, and after completion of the warming-upoperation of the internal combustion engine, is switched so as to causeall the exhaust to flow through the NOx absorbing material at a point intime when the amount of NOx discharged from the internal combustionengine has become smaller than a predetermined amount.
 10. An exhaustemission purifying device of an internal combustion engine according toclaim 9, wherein when the amount of NOx discharged from the internalcombustion engine is smaller than a predetermined amount, the vehicle inwhich the internal combustion is mounted is running at a deceleratedspeed.
 11. An exhaust emission purifying device of an internalcombustion engine according to claim 9, wherein when the amount of NOxdischarged from the internal combustion engine is smaller than apredetermined amount, the load of the internal combustion engine is lessthan a predetermined value.
 12. An exhaust emission purifying device ofan internal combustion engine, comprising: a lean-burn type internalcombustion engine capable of burning an air-fuel mixture with excessiveoxygen; an NOx absorbing material which is arranged in an exhaustpassage of the internal combustion engine and which is adapted to absorbnitrogen oxides in the exhaust when an air-fuel ratio of the exhaustflowing is lean and to release the nitrogen oxides it has absorbed whenthe oxygen concentration of the exhaust flowing in is low; a bypass pathbranching off from the portion of the exhaust passage on the upstreamside of the NOx absorbing material and adapted to cause the exhaust tobypass the NOx absorbing material; an exhaust flow switching means forselectively switching the flow of the exhaust between the NOx absorbingmaterial and the bypass path; an SOx absorbing material which isarranged in the portion of the exhaust passage on the upstream side ofthe exhaust flow switching means and which is adapted to absorb sulfuroxides when the air-fuel ratio of the exhaust flowing-in is lean and torelease the sulfur oxides it has absorbed when the oxygen concentrationof the exhaust flowing in is low; and an NOx catalyst provided in thebypass path and adapted to purify nitrogen oxides when the air-fuelratio of the exhaust flowing in is lean, wherein the NOx catalystprovided in the bypass path is an occlusion reduction type NOx catalystwhich absorbs nitrogen oxides in the exhaust when the air-fuel ratio ofthe exhaust is lean and which releases the nitrogen oxides it hasabsorbed to reduce or decompose them when the oxygen concentration ofthe exhaust is less and there exists a reducing agent, furthercomprising an SOx poisoning regenerative means which, when SOx poisoningof at least one of the NOx absorbing material and the NOx catalyst isdetected, controls the exhaust flow switching means so as to cause allthe exhaust to flow through both the NOx absorbing material and the NOxcatalyst and executes the SOx poisoning regenerative processingsimultaneously on the NOx absorbing material and the NOx catalyst. 13.An exhaust emission purifying device of an internal combustion engineaccording to claim 12, further comprising a regeneration completiondetermination means for determining the completion of the SOx poisoningregeneration of the NOx absorbing material and the NOx catalyst, andwherein when it is determined by the regeneration completiondetermination means that the SOx poisoning regeneration of one of theNOx absorbing material and the NOx catalyst has been completed, the SOxpoisoning regeneration means controls the exhaust flow switching meansso as to prevent the exhaust from flowing into the substance of whichthe SOx poisoning regeneration has been completed.
 14. An exhaustemission purifying device of an internal combustion engine according toclaim 12, further comprising a regeneration completion determinationmeans for determining the completion of the SOx poisoning regenerationof the NOx absorbing material and the NOx catalyst, wherein when it isdetermined by the regeneration completion determination means that theSOx poisoning regeneration of one of the NOx absorbing material and theNOx catalyst has been completed, the SOx poisoning regeneration meansinterrupts the SOx poisoning regeneration processing, and cools thesubstance of which the SOx poisoning regeneration has been completed,and wherein after the completion of the cooling of the substance ofwhich the SOx poisoning regeneration has been completed, SOx poisoningregeneration processing is resumed only on the substance of which theSOx poisoning regeneration has not been completed yet.
 15. An exhaustemission purifying device of an internal combustion engine, comprising:a lean-burn type internal combustion engine capable of burning anair-fuel mixture with excessive oxygen; an NOx absorbing material whichis arranged in an exhaust passage of the internal combustion engine andwhich is adapted to absorb nitrogen oxides in the exhaust when anair-fuel ratio of the exhaust flowing is lean and to release thenitrogen oxides it has absorbed when the oxygen concentration of theexhaust flowing in is low; a bypass path branching off from the portionof the exhaust passage on the upstream side of the NOx absorbingmaterial and adapted to cause the exhaust to bypass the NOx absorbingmaterial; an exhaust flow switching means for selectively switching theflow of the exhaust between the NOx absorbing material and the bypasspath; an SOx absorbing material which is arranged in the portion of theexhaust passage on the upstream side of the exhaust flow switching meansand which is adapted to absorb sulfur oxides when the air-fuel ratio ofthe exhaust flowing-in is lean and to release the sulfur oxides it hasabsorbed when the oxygen concentration of the exhaust flowing in is low;and an NOx catalyst provided in the bypass path and adapted to purifynitrogen oxides when the air-fuel ratio of the exhaust flowing in islean, wherein the NOx catalyst provided in the bypass path is anocclusion reduction type NOx catalyst which absorbs nitrogen oxides inthe exhaust when the air-fuel ratio of the exhaust is lean and whichreleases the nitrogen oxides it has absorbed to reduce or decompose themwhen the oxygen concentration of the exhaust is less and there exists areducing agent, further comprising an NOx absorption amount detectionmeans for detecting the amount of NOx absorbed by the NOx absorbingmaterial and the amount of NOx absorbed by the NOx catalyst, wherein theNOx absorption amount detection means estimates the NOx absorptionamount of the NOx absorbing material and of the NOx catalyst on thebasis of the amount of exhaust leaking from the exhaust flow switchingmeans.
 16. An exhaust emission purifying device of an internalcombustion engine, comprising: a lean-burn type internal combustionengine capable of burning an air-fuel mixture with excessive oxygen; anNOx absorbing material which is arranged in an exhaust passage of theinternal combustion engine and which is adapted to absorb nitrogenoxides in the exhaust when an air-fuel ratio of the exhaust flowing islean and to release the nitrogen oxides it has absorbed when the oxygenconcentration of the exhaust flowing in is low; a bypass path branchingoff from the portion of the exhaust passage on the upstream side of theNOx absorbing material and adapted to cause the exhaust to bypass theNOx absorbing material; an exhaust flow switching means for selectivelyswitching the flow of the exhaust between the NOx absorbing material andthe bypass path; an SOx absorbing material which is arranged in theportion of the exhaust passage on the upstream side of the exhaust flowswitching means and which is adapted to absorb sulfur oxides when theair-fuel ratio of the exhaust flowing-in is lean and to release thesulfur oxides it has absorbed when the oxygen concentration of theexhaust flowing in is low; and an NOx catalyst provided in the bypasspath and adapted to purify nitrogen oxides when the air-fuel ratio ofthe exhaust flowing in is lean, wherein the NOx catalyst provided in thebypass path is an occlusion reduction type NOx catalyst which absorbsnitrogen oxides in the exhaust when the air-fuel ratio of the exhaust islean and which releases the nitrogen oxides it has absorbed to reduce ordecompose them when the oxygen concentration of the exhaust is leselessand there exists a reducing agent, further comprising an NOx purifyingmeans which, when it becomes necessary to control the exhaust flowswitching means so as to cause the exhaust to flow through both the NOxabsorbing material and the NOx catalyst, controls the exhaust flowswitching means so as to cause the exhaust to flow through both the NOxabsorbing material and the NOx catalyst after releasing and purifyingall the NOx absorbed by the NOx absorbing material and the NOx catalyst.17. An exhaust emission purifying device of an internal combustionengine according to claim 16, wherein when the exhaust flow switchingmeans is controlled so as to cause the exhaust to flow through both theNOx absorbing material and the NOx catalyst, the NOx purifying meanssimultaneously releases and purifies the NOx absorbed by the NOxabsorbing material and the NOx absorbed by the NOx catalyst, using, ofthe NOx absorbing material and the NOx catalyst, the one whose NOxabsorbing capacity is lower as a reference.
 18. An exhaust emissionpurifying device of an internal combustion engine, comprising: alean-burn type internal combustion engine capable of burning an air-fuelmixture with excessive oxygen; an NOx absorbing material which isarranged in an exhaust passage of the internal combustion engine andwhich is adapted to absorb nitrogen oxides in the exhaust when anair-fuel ratio of the exhaust flowing is lean and to release thenitrogen oxides it has absorbed when the oxygen concentration of theexhaust flowing in is low; a bypass path branching off from the portionof the exhaust passage on the upstream side of the NOx absorbingmaterial and adapted to cause the exhaust to bypass the NOx absorbingmaterial; an exhaust flow switching means for selectively switching theflow of the exhaust between the NOx absorbing material and the bypasspath; an SOx absorbing material which is arranged in the portion of theexhaust passage on the upstream side of the exhaust flow switching meansand which is adapted to absorb sulfur oxides when the air-fuel ratio ofthe exhaust flowing-in is lean and to release the sulfur oxides it hasabsorbed when the oxygen concentration of the exhaust flowing in is low;and an NOx catalyst provided in the bypass path and adapted to purifynitrogen oxides when the air-fuel ratio of the exhaust flowing in islean, wherein the exhaust flow switching means is controlled so as tocause the exhaust to be led to the NOx catalyst and as to prevent theexhaust from flowing into the NOx absorbing material when during thewarming-up operation of the internal combustion engine the air-fuelratio of the exhaust is being controlled to be stoichiometric or rich,and is controlled so as to cause the exhaust to be led to the NOxabsorbing material and to prevent the exhaust from flowing into the NOxcatalyst when, during the warning-up operation of the internalcombustion engine, the amount of NOx discharged from the internalcombustion engine is smaller than a predetermined amount.
 19. An exhaustemission purifying device of an internal combustion engine, comprising:a lean-burn type internal combustion engine capable of burning anair-fuel mixture with excessive oxygen; an NOx absorbing material whichis provided in an exhaust passage of the internal combustion engine andwhich is adapted to absorb nitrogen oxides in the exhaust when theair-fuel ratio of the exhaust flowing is lean and to release thenitrogen oxides it has absorbed when the oxygen concentration of theexhaust flowing in is low; a bypass path branching off from the portionof the exhaust passage on the upstream side of the NOx absorbingmaterial and adapted to cause the exhaust to bypass the NOx absorbingmaterial; an exhaust flow switching means for selectively switching theflow of the exhaust between the NOx absorbing material and the bypasspath; an SOx absorbing material which is arranged in the portion of theexhaust passage on the upstream side of the exhaust flow switching meansand which is adapted to absorb sulfur oxides when the air-fuel ratio ofthe exhaust flowing-in is lean and to release the sulfur oxides it hasabsorbed when the oxygen concentration of the exhaust flowing in is low;and an NOx catalyst provided in the exhaust passage on the downstreamside of the bypass path and adapted to purify nitrogen oxides when theair-fuel ratio of the exhaust is lean; wherein the exhaust flowswitching means causes all the exhaust to flow through the NOx absorbingmaterial when the air-fuel ratio of the exhaust is controlled to belean, and causes all the exhaust to flow through the bypass path whenthe air-fuel ratio of the exhaust is controlled to be stoichiometric orrich.